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2001-07-19
|
The Altitude of an Infrared Bright Cloud Feature on Neptune from Near-Infrared Spectroscopy
|
We present 2.03-2.30 micron near-infrared spectroscopy of Neptune taken 1999
June 2 (UT) with the W.M. Keck Observatory's near-infrared spectrometer
(NIRSPEC) during the commissioning of the instrument. The spectrum is dominated
by a bright cloud feature, possibly a storm or upwelling, in the southern
hemisphere at approximately 50 degrees S latitude. The spectrum also includes
light from a dimmer northern feature at approximately 30 degrees N latitude. We
compare our spectra (R ~ 2000) of these two features with a simple model of
Neptune's atmosphere. Given our model assumption that the clouds are flat
reflecting layers, we find that the top of the bright southern cloud feature
sat at a pressure level of 0.14 (+0.05, -0.03) bar, and thus this cloud did not
extend into the stratosphere (P < 0.1 bar). A similar analysis of the dimmer
northern feature gives a cloud-top pressure of 0.084 +/- 0.026 bar. This
suggests that the features we observed efficiently transport methane to the
base of the stratosphere, but do not directly transport methane to the upper
stratosphere (P < 0.01-0.001 bar) where photolysis occurs. Our observations do
not constrain how far these clouds penetrate down into the troposphere. We find
that our model fits to the data restrict the fraction of molecular hydrogen in
ortho/para thermodynamic equilibrium to greater than 0.8.
|
0107361v1
|
2002-03-20
|
Observation of X-ray variability in the BL Lac object 1ES1959+65
|
This paper reports X-ray spectral observations of a relatively nearby z =
0.048 BL Lacertae (BL Lac) object 1ES1959+650, which is a potential TeV
emitter. The observations include 31 short pointings made by the Unconventional
Stellar Aspect (USA) Experiment on board the Advanced Research and Global
Observation Satellite (ARGOS), and 17 pointings by the PCA on board the Rossi
X-ray Timing Explorer (RXTE). Most of these observations were spaced by less
than 1 day. \es was detected by the ARGOS USA detector in the range 1-16 keV,
and by the PCA in the 2-16 keV range but at different times. During the closely
spaced RXTE observations beginning on 2000 July 28, an ending of one flare and
a start of another are visible, associated with spectral changes, where the
photon index Gamma ranges between ~ 1.4 and 1.7, and the spectrum is harder
when the source is brighter. This implies that 1ES1959 is an XBL-type blazar,
with the X-ray emission likely to originate via the synchrotron process. The
USA observations reveal another flare that peaked on 2000 November 14 and
doubled the flux within a few days, again associated with spectral changes of
the same form. The spectral variability correlated with the flux and timing
characteristics of this object that are similar to those of other nearby BL
Lacs, and suggest relativistic beaming with a Doppler factor delta > 1.6 and
magnetic fields of the order of a few mG. We also suggest that the steady
component of the X-ray emission -- present in this object as well as in other
XBLs -- may be due to the large-scale relativistic jet (such as measured by
Chandra in many radio-loud AGN), but pointing very closely to our line of
sight.
|
0203335v1
|
2002-06-18
|
Near-IR and Optical Morphology of Spiral Galaxies
|
We announce the initial release of data from the Ohio State University Bright
Spiral Galaxy Survey, a BVRJHK imaging survey of a well-defined sample of 205
bright, nearby spiral galaxies. We present H-band morphological classification
on the Hubble sequence for the OSU Survey sample. We compare the H-band
classification to B-band classification from our own images and from standard
galaxy catalogs. Our B-band classifications match well with those of the
standard catalogs. On average, galaxies with optical classifications from Sa
through Scd appear about one T-type earlier in the H-band than in the B-band,
but with large scatter. This result does not support recent claims made in the
literature that the optical and near-IR morphologies of spiral galaxies are
uncorrelated. We present detailed descriptions of the H-band morphologies of
our entire sample, as well as B- and H-band images for a set of 17 galaxies
chosen as type examples, and BRH color-composite images of six galaxies chosen
to demonstrate the range in morphological variation as a function of
wavelength. Data from the survey are accessible at
http://www.astronomy.ohio-state.edu/~survey/
|
0206320v1
|
2003-02-20
|
The Reionization History at High Redshifts I: Physical Models and New Constraints from CMB Polarization
|
The recent discovery of a high optical depth tau to Thomson scattering from
the WMAP data implies that significant reionization took place at redshifts
z~15. This discovery has important implications for the sources of
reionization, and allows, for the first time, constraints to be placed on
physical reionization scenarios out to redshift z~20. Using a new suite of
semi-analytic reionization models, we show that the high value of tau requires
a surprisingly high efficiency epsilon of the first generation of UV sources
for injecting ionizing photons into the intergalactic medium. We find that no
simple reionization model can be consistent with the combination of the WMAP
result with data from the z<6.5 universe. Satisfying both constraints requires
either of the following: (i) H_2 molecules form efficiently at z~20, survive
feedback processes, and allow UV sources in halos with virial temperatures
below Tvir=10^4 K to contribute substantially to reionization, or (ii) the
efficiency epsilon in halos with Tvir>10^4K decreased by a factor of ~ 30
between (z~20) and (z~6). We discuss the relevant physical issues to produce
either scenario, and argue that both options are viable, and allowed by current
data. In detailed models of the reionization history, we find that the
evolution of the ionized fractions in the two scenarios have distinctive
features that Planck can distinguish at 3 sigma significance. At the high WMAP
value for tau, Planck will also be able to provide tight statistical
constraints on reionization model parameters, and elucidate much of the physics
at the end of the Dark Ages. The sources responsible for the high optical depth
discovered by WMAP should be directly detectable out to z~15 by the James Webb
Space Telescope.
|
0302403v2
|
2004-06-07
|
Dynamics and Stellar Content of the Giant Southern Stream in M31. I. Keck Spectroscopy of Red Giant Stars
|
We present results from a large spectroscopic survey of M31 red giants using
the Keck telescope/DEIMOS. Photometric pre-screening, based on the 100A-wide
DDO51 band centered on the Mgb/MgH feature, was used to select spectroscopic
targets. Red giant candidates were targeted in a small field on M31's giant
southern tidal stream at a projected distance of 31kpc from the galaxy center.
We isolate a clean sample of 68 giants by removing contaminants (foreground
Galactic dwarfs and background galaxies) using spectroscopic, imaging, and
photometric methods. About 65% of the M31 stars are found to be members of the
stream, while the rest appear to be members of the general halo population. The
mean (heliocentric) radial velocity of the stream in our field is -458 km/s, or
-158 km/s relative to M31's systemic velocity, in good agreement with recent
measurements at other stream locations. The intrinsic velocity dispersion of
the stream is constrained to be 15_{-15}^{+8} km/s (90% confidence limits). The
companion paper by Font et al. (2004, astro-ph/0406146) discusses possible
orbits, implications of the coldness of the stream, and progenitor satellite
properties. The kinematics (and perhaps [Fe/H] distribution) of our halo sample
indicate that it is different from other M31 halo samples; this may be an
indication of substructure in the halo. The stream seems to have a higher mean
[Fe/H] than the halo, -0.51 vs -0.74 dex, and a smaller [Fe/H] spread. The
stream's high metallicity implies that its progenitor must have been a luminous
dwarf galaxy. The CaII triplet strengths of the M31 giants are generally
consistent with photometric estimates of their metallicity (derived by fitting
RGB fiducials in the color-magnitude diagram). There is indirect evidence of
intermediate-age stars in the stream.
|
0406145v1
|
2004-07-14
|
Central Masses and Broad-Line Region Sizes of Active Galactic Nuclei. II. A Homogeneous Analysis of a Large Reverberation-Mapping Database
|
We present improved black hole masses for 35 active galactic nuclei (AGNs)
based on a complete and consistent reanalysis of broad emission-line
reverberation-mapping data. From objects with multiple line measurements, we
find that the highest precision measure of the virial product is obtained by
using the cross-correlation function centroid (as opposed to the
cross-correlation function peak) for the time delay and the line dispersion (as
opposed to full width half maximum) for the line width and by measuring the
line width in the variable part of the spectrum. Accurate line-width
measurement depends critically on avoiding contaminating features, in
particular the narrow components of the emission lines. We find that the
precision (or random component of the error) of reverberation-based black hole
mass measurements is typically around 30%, comparable to the precision attained
in measurement of black hole masses in quiescent galaxies by gas or stellar
dynamical methods. Based on results presented in a companion paper by Onken et
al., we provide a zero-point calibration for the reverberation-based black hole
mass scale by using the relationship between black hole mass and host-galaxy
bulge velocity dispersion. The scatter around this relationship implies that
the typical systematic uncertainties in reverberation-based black hole masses
are smaller than a factor of three. We present a preliminary version of a
mass-luminosity relationship that is much better defined than any previous
attempt. Scatter about the mass-luminosity relationship for these AGNs appears
to be real and could be correlated with either Eddington ratio or object
inclination.
|
0407299v1
|
2005-02-06
|
SINFONI in the Galactic Center: young stars and IR flares in the central light month
|
We report 75 milli-arcsec resolution, near-IR imaging spectroscopy within the
central 30 light days of the Galactic Center [...]. To a limiting magnitude of
K~16, 9 of 10 stars in the central 0.4 arcsec, and 13 of 17 stars out to 0.7
arcsec from the central black hole have spectral properties of B0-B9, main
sequence stars. [...] all brighter early type stars have normal rotation
velocities, similar to solar neighborhood stars. We [...] derive improved 3d
stellar orbits for six of these S-stars in the central 0.5 arcsec. Their
orientations in space appear random. Their orbital planes are not co-aligned
with those of the two disks of massive young stars 1-10 arcsec from SgrA*. We
can thus exclude [...] that the S-stars as a group inhabit the inner regions of
these disks. They also cannot have been located/formed in these disks [...].
[...] we conclude that the S-stars were most likely brought into the central
light month by strong individual scattering events. The updated estimate of
distance to the Galactic center from the S2 orbit fit is Ro = 7.62 +/- 0.32
kpc, resulting in a central mass value of 3.61 +/- 0.32 x 10^6 Msun. We
happened to catch two smaller flaring events from SgrA* [...]. The 1.7-2.45 mum
spectral energy distributions of these flares are fit by a featureless, red
power law [...]. The observed spectral slope is in good agreement with
synchrotron models in which the infrared emission comes from [...] radiative
inefficient accretion flow in the central R~10 Rs region.
|
0502129v1
|
2005-02-18
|
Discovery of an extended halo of metal-poor stars in the Andromeda spiral galaxy
|
Understanding galaxy formation involves look-back and fossil-record studies
of distant and nearby galaxies, respectively. Debris trails in our Galaxy's
spheroidal halo of old stars provide evidence of "bottom-up" formation via
tidal disruption/merging of dwarf satellite galaxies, but it is difficult to
study our Galaxy's large-scale structure from within. Studies of our
neighbouring Andromeda galaxy have concluded that its spheroid contains
chemically enriched ("metal-rich") stars out to a radius of 30 kiloparsecs with
an exponential r^1/4 fall-off in density thereby resembling a galactic "bulge".
Were Andromeda's true halo to be found, our detailed yet global view of its
stellar dynamics, substructure, chemical abundance, and age distribution would
directly constrain hierarchical halo formation models. We report here on the
discovery of a hitherto elusive halo of metal-poor Andromeda stars, distinct
from its bulge, with a power-law brightness profile extending beyond r = 160
kiloparsecs. This is 3 - 5 times larger than any previously mapped Andromeda
spheroidal/disk component. Together, the Galactic and Andromeda halos span >1/3
of the distance between them, suggesting that stars occupy a substantial volume
fraction of our Local Group, and possibly most galaxy groups.
|
0502366v5
|
2005-04-12
|
Constrained Cluster Parameters from Sunyaev-Zel'dovich Observations
|
Near-future SZ surveys such as ACT, SPT, APEX, and Planck will find thousands
of galaxy clusters. Multi-frequency arcminute-resolution SZ observations can,
in principle, determine each cluster's gas temperature (T_e), bulk velocity
(v_pec), and optical depth (tau). However, the frequency bands and detector
sensitivity employed by upcoming surveys will generally not be sufficient to
disentangle the degeneracy between these three cluster parameters, even in the
absence of SZ signal contamination from point sources and imperfect primary
microwave background subtraction. Assuming contaminants can be removed, we find
that near-future SZ surveys will be able to constrain well two cluster gas
parameters that are linear combinations of tau*T_e, tau*v_z, and tau*T_e^2.
Because the SZ intensity shift is nearly a linear function of tau*T_e, tau*v_z,
and tau*T_e^2, a correspondence exists between the two effective gas parameters
that SZ surveys can constrain and simple line-of-sight integrals through the
three dimensional cluster. We illustrate the parameter constraints and
correspondence to line-of-sight integrals using 3D Nbody + hydro cluster
simulations and a Markov chain Monte Carlo method. We show that adding an
independent T_e measurement to upcoming SZ data breaks the parameter
degeneracy. The cluster effective velocity thus constrained is approximately
the optical-depth-weighted velocity integrated along the cluster line of sight.
A temperature prior with an error as large as 2 keV still gives bulk velocity
errors < 100 km/sec, even for a more typical cluster with an electron
temperature of 3 keV, for ACT-like SZ observations in the absence of signal
contamination. The Markov chain constraints on v_pec and tau are more
encouraging and likely more accurate than those obtained from Fisher matrices.
|
0504274v3
|
2005-10-03
|
High Spectral Resolution Observations of the Massive Stars in the Galactic Center
|
We present high-resolution near-infrared spectra, obtained with the NIRSPEC
spectrograph on the W. M. Keck II Telescope, of a collection of hot, massive
stars within the central 25 arcseconds of the Galactic center. We have
identified a total of twenty-one emission-line stars, seven of which are new
radial velocity detections with five of those being classified as He I
emission-line stars for the first time. These stars fall into two categories
based on their spectral properties: 1) those with narrow 2.112, 2.113 micron He
I doublet absorption lines, and 2) those with broad 2.058 micron He I emission
lines. These data have the highest spectral resolution ever obtained for these
sources and, as a result, both components of the absorption doublet are
separately resolved for the first time. We use these spectral features to
measure radial velocities. The majority of the measured radial velocities have
relative errors of 20 kms, smaller than those previously obtained with
proper-motion or radial velocity measurements for similar stellar samples in
the Galactic center. The radial velocities estimated from the He I absorption
doublet are more robust than those previously estimated from the 2.058 micron
emission line, since they do not suffer from confusion due to emission from the
surrounding ISM. Using this velocity information, we agree that the stars are
orbiting in a somewhat coherent manner but are not as defined into a disk or
disks as previously thought. Finally, multi-epoch radial velocity measurements
for IRS 16NE show a change in its velocity presumably due to an unseen stellar
companion.
|
0510028v2
|
2005-12-06
|
Kinematics and Metallicity of M31 Red Giants: The Giant Southern Stream and Discovery of a Second Cold Component at R = 20 kpc
|
We present spectroscopic observations of red giant branch (RGB) stars in the
Andromeda spiral galaxy (M31), acquired with the DEIMOS instrument on the Keck
II 10-m telescope. The three fields targeted in this study are in the M31
spheroid, outer disk, and giant southern stream. In this paper, we focus on the
kinematics and chemical composition of RGB stars in the stream field located at
a projected distance of R = 20 kpc from M31's center. A mix of stellar
populations is found in this field. M31 RGB stars are isolated from Milky Way
dwarf star contaminants using a variety of spectral and photometric
diagnostics. The radial velocity distribution of RGB stars displays a clear
bimodality -- a primary peak centered at v = -513 km/s and a secondary one at v
= -417 km/s -- along with an underlying broad component that is presumably
representative of the smooth spheroid of M31. Both peaks are found to be
dynamically cold with intrinsic velocity dispersions of sigma(v) = 16 km/s. The
mean metallicity and metallicity dispersion of stars in the two peaks is also
found to be similar: [Fe/H] = -0.45 and sigma([Fe/H]) = 0.2. The observed
velocity of the primary peak is consistent with that predicted by dynamical
models for the stream, but there is no obvious explanation for the secondary
peak. The nature of the secondary cold population is unclear: it may represent:
(1) tidal debris from a satellite merger event that is superimposed on, but
unrelated to, the giant southern stream; (2) a wrapped around component of the
giant southern stream; (3) a warp or overdensity in M31's disk at R > 50 kpc
(this component is well above the outward extrapolation of the smooth
exponential disk brightness profile).
|
0512161v1
|
2006-02-28
|
Rejecting Astrophysical False Positives from the TrES Transiting Planet Survey: The Example of GSC 03885-00829
|
Ground-based wide-field surveys for nearby transiting gas giants are yielding
far fewer true planets than astrophysical false positives, of which some are
difficult to reject. Recent experience has highlighted the need for careful
analysis to eliminate astronomical systems where light from a faint eclipsing
binary is blended with that from a bright star. During the course of the
Trans-atlantic Exoplanet Survey, we identified a system presenting a
transit-like periodic signal. We obtained the proper motion and infrared color
of this target (GSC 03885-00829) from publicly available catalogs, which
suggested this star is an F dwarf, supporting our transit hypothesis. This
spectral classification was confirmed using spectroscopic observations from
which we determined the stellar radial velocity. The star did not exhibit any
signs of a stellar mass companion. However, subsequent multi-color photometry
displayed a color-dependent transit depth, indicating that a blend was the
likely source of the eclipse. We successfully modeled our initial photometric
observations of GSC 03885-00829 as the light from a K dwarf binary system
superimposed on the light from a late F dwarf star. High-dispersion
spectroscopy confirmed the presence of light from a cool stellar photosphere in
the spectrum of this system. With this candidate, we demonstrate both the
difficulty in identifying certain types of false positives in a list of
candidate transiting planets and our procedure for rejecting these imposters,
which may be useful to other groups performing wide-field transit surveys.
|
0603005v1
|
2006-03-21
|
SINFONI Integral Field Spectroscopy of z~2 UV-selected Galaxies: Rotation Curves and Dynamical Evolution
|
We present 0.5" resolution near-IR integral field spectroscopy of the Ha line
emission of 14 z~2 UV-selected BM/BX galaxies obtained with SINFONI at ESO/VLT.
The mean Ha half-light radius r_1/2 is about 4kpc and line emission is detected
over > ~20kpc in several sources. In 9 sources, we detect spatially-resolved
velocity gradients, from 40 to 410 km/s over ~10kpc. The observed kinematics of
the larger systems are consistent with orbital motions. Four galaxies are well
described by rotating disks with clumpy morphologies and we extract rotation
curves out to radii > ~10kpc. One or two galaxies exhibit signatures more
consistent with mergers. Analyzing all 14 galaxies in the framework of rotating
disks, we infer mean inclination- and beam-corrected maximum circular
velocities v_c of 180+-90 km/s and dynamical masses of (0.5-25)x10^10 Msun
within r_1/2. On average, the dynamical masses are consistent with photometric
stellar masses assuming a Chabrier/Kroupa IMF but too small for a 0.1-100 Msun
Salpeter IMF. The specific angular momenta of our BM/BX galaxies are similar to
those of local late-type galaxies. The specific angular momenta of their
baryons are comparable to those of their dark matter halos. Extrapolating from
the average v_c at 10kpc, the virial mass of the typical halo of a galaxy in
our sample is 10^(11.7+-0.5) Msun. Kinematic modeling of the 3 best cases
implies a ratio of v_c to local velocity dispersion of order 2-4 and
accordingly a large geometric thickness. We argue that this suggests a mass
accretion (alternatively, gas exhaustion) timescale of ~500Myr. We also argue
that if our BM/BX galaxies were initially gas rich, their clumpy disks will
subsequently lose their angular momentum and form compact bulges on a timescale
of ~1 Gyr. [ABRIDGED]
|
0603559v1
|
2006-05-06
|
The Metal-Poor Halo of the Andromeda Spiral Galaxy (M31)
|
We present spectroscopic observations of red giant branch (RGB) stars over a
large expanse in the halo of the Andromeda spiral galaxy (M31), acquired with
the DEIMOS instrument on the Keck II 10-m telescope. Using a combination of
five photometric/spectroscopic diagnostics -- (1) radial velocity, (2)
intermediate-width DDO51 photometry, (3) Na I equivalent width (surface gravity
sensitive), (4) position in the color-magnitude diagram, and (5) comparison
between photometric and spectroscopic [Fe/H] estimates -- we isolate over 250
bona fide M31 bulge and halo RGB stars located in twelve fields ranging from R
= 12-165kpc from the center of M31 (47 of these stars are halo members with R >
60 kpc). We derive the photometric and spectroscopic metallicity distribution
function of M31 RGB stars in each of these fields. The mean of the resulting
M31 spheroid (bulge and halo) metallicity distribution is found to be
systematically more metal-poor with increasing radius, shifting from <[Fe/H]> =
-0.47+/-0.03 (sigma = 0.39) at R < 20 kpc to <[Fe/H]> = -0.94+/-0.06 (sigma =
0.60) at R ~ 30 kpc to <[Fe/H]> = -1.26+/-0.10 (sigma = 0.72) at R > 60 kpc,
assuming [alpha/Fe] = 0.0. These results indicate the presence of a metal-poor
RGB population at large radial distances out to at least R = 160 kpc, thereby
supporting our recent discovery of a stellar halo in M31: its halo and bulge
(defined as the structural components with R^{-2} power law and de Vaucouleurs
R^{1/4} law surface brightness profiles, respectively) are shown to have
distinct metallicity distributions. If we assume an alpha-enhancement of
[alpha/Fe] = +0.3 for M31's halo, we derive <[Fe/H]> = -1.5+/-0.1 (sigma =
0.7). Therefore, the mean metallicity and metallicity spread of this newly
found remote M31 RGB population are similar to those of the Milky Way halo.
|
0605170v1
|
2006-05-08
|
A New Method for Isolating M31 Red Giant Stars: The Discovery of Stars out to a Radial Distance of 165 Kiloparsecs
|
We present a method for isolating a clean sample of red giant stars in the
outerregions of the Andromeda spiral galaxy (M31) from an ongoing spectroscopic
survey using the DEIMOS instrument on the Keck 10-m telescope. The survey aims
to study the kinematics, global structure, substructure, and metallicity of
M31's halo. Although most of our spectroscopic targets were photometrically
screened to reject foreground Milky Way dwarf star contaminants, the latter
class of objects still constitutes a substantial fraction of the observed
spectra in the sparse outer halo. Our likelihood-based method for isolating M31
red giants uses five criteria: (1) radial velocity, (2) photometry in the
intermediate-width DDO51 band to measure the strength of the MgH/Mgb absorption
features, (3) strength of the Na I 8190A absorption line doublet, (4) location
within an (I, V-I) color-magnitude diagram, and (5) comparison of photometric
(CMD-based) versus spectroscopic (Ca II 8500A triplet-based) metallicity
estimates. We also discuss K I and TiO diagnostics for giant/dwarf separation
that might be useful in future analyses. Training sets consisting of definite
M31 red giants and Galactic dwarf stars are used to derive empirical
probabilitydistribution functions for each diagnostic. These functions are used
to calculate the likelihood that a given star is a red giant in M31 versus a
Milky Way dwarf. By applying this diagnostic method to our spectroscopic data
set, we isolate 40 M31 red giants beyond a projected distance of R = 60 kpc
from the galaxy's center, including three out at R ~ 165 kpc. The ability to
identify individual M31 red giants gives us an unprecedented level of
sensitivity in studying the properties of the galaxy's outer halo.
|
0605171v2
|
2006-06-21
|
Extreme gas kinematics in the z=2.2 powerful radio galaxy MRC1138-262: Evidence for efficient AGN feedback in the early Universe?
|
To explain the properties of the most massive low-redshift galaxies and the
shape of their mass function, recent models of galaxy evolution include strong
AGN feedback to complement starburst-driven feedback in massive galaxies. Using
the near-infrared integral-field spectrograph SPIFFI on the VLT, we searched
for direct evidence for such a feedback in the optical emission line gas around
the z=2.16 powerful radio galaxy MRC1138-262, likely a massive galaxy in
formation. The kpc-scale kinematics, with FWHMs and relative velocities <= 2400
km/s and nearly spherical spatial distribution, do not resemble large-scale
gravitational motion or starburst-driven winds. Order-of-magnitude timescale
and energy arguments favor the AGN as the only plausible candidate to
accelerate the gas, with a total energy injection of a few x 10^60 ergs or
more, necessary to power the outflow, and relatively efficient coupling between
radio jet and ISM. Observed outflow properties are in gross agreement with the
models, and suggest that AGN winds might have a similar, or perhaps larger,
cosmological significance than starburst-driven winds, if MRC1138-262 is indeed
archetypal. Moreover, the outflow has the potential to remove significant gas
fractions (<= 50%) from a >L* galaxy within a few 10 to 100 Myrs, fast enough
to preserve the observed [alpha/Fe] overabundance in massive galaxies at low
redshift. Using simple arguments, it appears that feedback like that observed
in MRC1138-262 may have sufficient energy to inhibit material from infalling
into the dark matter halo and thus regulate galaxy growth as required in some
recent models of hierarchical structure formation.
|
0606530v1
|
2006-05-02
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Proton NMR measurements of the local magnetic field in the paramagnetic metal and antiferromagnetic insulator phases of $λ$-(BETS)$_{2}$FeCl$_{4}$
|
Measurements of the $^{1}$H-NMR spectrum of a small ($\sim$ 4 $\mu$g) single
crystal of the organic conductor $\lambda$-(BETS)$_{2}$FeCl$_{4}$ are reported
with an applied magnetic field $\bf{B}$$_{0}$ = 9 T parallel to the a-axis in
the $ac$-plane over a temperature $(T)$ range 2.0 $-$ 180 K. They provide the
distribution of the static local magnetic field at the proton sites in the
paramagnetic metal (PM) and antiferromagnetic insulator (AFI) phases, along
with the changes that occur at the PM$-$AFI phase transition. The spectra have
six main peaks that are significantly broadened and shifted at low $T$. The
origin of these features is attributed to the large dipolar field from the 3d
Fe$^{3+}$ ion moments (spin $S_{\rm{d}}$ = 5/2). Their amplitude and
$T-$dependence are modeled using a modified Brillouin function that includes a
mean field approximation for the total exchange interaction ($J_{0}$) between
one Fe$^{3+}$ ion and its two nearest neighbors. A good fit is obtained using
$J_{0}$ = $-$ 1.7 K. At temperatures below the PM$-$AFI transition temperature
$T_{MI}$ = 3.5 K, an extra peak appears on the high frequency side of the
spectrum and the details of the spectrum become smeared. Also, the rms
linewidth and the frequency shift of the spectral distribution are
discontinuous, consistent with the transition being first-order. These
measurements verify that the dominant local magnetic field contribution is from
the Fe$^{3+}$ ions and indicate that there is a significant change in the
static local magnetic field distribution at the proton sites on traversing the
PM to AFI phase transition.
|
0605044v2
|
2006-09-07
|
$^{1}$H-NMR spin-echo measurements of the static and dynamic spin properties in $λ$-(BETS)$_{2}$FeCl$_{4}$
|
$^{1}$H-NMR spin-echo measurements of the spin-echo decay $M(2\tau)$ with a
decay rate 1/$T_{2}$ and the frequency shift $\Delta\nu/\nu_{0}$ under applied
magnetic field $\mathbf{B}$$_{0}$ = 9 T along the a-axis over a temperature
range 2.0$-$180 K are reported for a single crystal of the organic conductor
$\lambda$-(BETS)$_{2}$FeCl$_{4}$. It provides the spin dynamic and static
properties in the paramagnetic metal (PM) and antiferromagnetic insulator (AFI)
states as well as across the PM$-$AFI phase transition. A large slow beat
structure in the spin-echo decay is observed with a typical beat frequency of
$f$ $\sim$ 7 kHz and it varies across the spectrum. Its origin is attributed to
the $^{1}$H$-$$^{1}$H dipole interactions rather than to the much larger
dipolar field contribution from the Fe$^{3+}$ electrons (spin $S$ = 5/2). A
simple phenomenological model provides an excellent fit to the data. The
dominant $^{1}$H-NMR frequency shift comes from the dipolar field from the 3d
Fe$^{3+}$ ions, and the Fe$^{3+}$ $-$ Fe$^{3+}$ exchange interactions ($J_{0}$)
($J_{0} $ includes the d$-$d exchange interactions through the $\pi-$electrons)
have a substantial effect to the local field at the proton sites expecially at
low temperatures. A good fit is obtained with $J_{0}$ = - 1.7 K. The data of
the spin-echo decay rate 1/$T_{2}$ indicates that there is a significant change
in the slow fluctuations of the local magnetic field at the $^{1}$H-sites on
traversing the PM to AFI phase. This evidence supports earlier reports that the
PM$-$AFI phase transition in $\lambda$-(BETS)$_{2}% $FeCl$_{4} $ is driven
magnetically and first order.
|
0609151v2
|
2007-05-14
|
TrES-3: A Nearby, Massive, Transiting Hot Jupiter in a 31-Hour Orbit
|
We describe the discovery of a massive transiting hot Jupiter with a very
short orbital period (1.30619 d), which we name TrES-3. From spectroscopy of
the host star GSC 03089-00929, we measure T_eff = 5720 +- 150 K, logg=4.6 +-
0.3, and vsini < 2 km/s, and derive a stellar mass of 0.90 +- 0.15 M_sun. We
estimate a planetary mass of 1.92 +- 0.23 M_Jup, based on the sinusoidal
variation of our high-precision radial velocity measurements. This variation
has a period and phase consistent with our transit photometry. Our spectra show
no evidence of line bisector variations that would indicate a blended eclipsing
binary star. From detailed modeling of our B and z photometry of the 2.5%-deep
transits, we determine a stellar radius 0.802 +- 0.046 R_sun and a planetary
radius 1.295 +- 0.081 R_Jup. TrES-3 has one of the shortest orbital periods of
the known transiting exoplanets, facilitating studies of orbital decay and mass
loss due to evaporation, and making it an excellent target for future studies
of infrared emission and reflected starlight.
|
0705.2004v1
|
2007-08-06
|
TrES-4: A Transiting Hot Jupiter of Very Low Density
|
We report the discovery of TrES-4, a hot Jupiter that transits the star GSC
02620-00648 every 3.55 days. From high-resolution spectroscopy of the star we
estimate a stellar effective temperature of Teff = 6100 +/- 150 K, and from
high-precision z and B photometry of the transit we constrain the ratio of the
semi-major axis and the stellar radius to be a/R = 6.03 +/- 0.13. We compare
these values to model stellar isochrones to constrain the stellar mass to be M*
= 1.22 +/- 0.17 Msun. Based on this estimate and the photometric time series,
we constrain the stellar radius to be R* = 1.738 +/- 0.092 Rsun and the planet
radius to be Rp = 1.674 +/- 0.094 RJup. We model our radial-velocity data
assuming a circular orbit and find a planetary mass of 0.84 +/- 0.10 MJup. Our
radial-velocity observations rule out line-bisector variations that would
indicate a specious detection resulting from a blend of an eclipsing binary
system. TrES-4 has the largest radius and lowest density of any of the known
transiting planets. It presents a challenge to current models of the physical
structure of hot Jupiters, and indicates that the diversity of physical
properties amongst the members of this class of exoplanets has yet to be fully
explored.
|
0708.0834v1
|
2007-08-30
|
Compact radio sources and jet-driven AGN feedback in the early Universe: Constraints from integral-field spectroscopy
|
To investigate the impact of radio jets during the formation epoch of their
massive host galaxies, we present an analysis of two massive, log(M_stel/
M_sun)~10.6 and 11.3, compact radio galaxies at z=3.5, TNJ0205+2242 and
TNJ0121+1320. Their small radio sizes (R<= 10 kpc) are most likely a sign of
youth. We compare their radio properties and gas dynamics with those in well
extended radio galaxies at high redshift, which show strong evidence for
powerful, jet-driven outflows of significant gas masses (M 10^9-10 M_sun). Our
analysis combines rest-frame optical integral-field spectroscopy with existing
radio imaging, CO emission line spectra, and rest-frame UV spectroscopy.
[OIII]5007 line emission is compact in both galaxies and lies within the region
defined by the radio lobes. For TNJ0205+2242, the Ly-alpha profile narrows
significantly outside the jet radius, indicating the presence of a quiescent
halo. TNJ0121+1320 has two components separated by ~10 kpc and a velocity
offset of ~300 km s^-1. If motions are gravitational, this implies a dynamical
mass of 2x10^11 M_sun for the more massive, radio-loud component. The dynamical
mass, molecular gas mass measured from the CO line emission, and radio
luminosity of these two compact radio galaxies imply that compact radio sources
may well develop large-scale, energetic outflows as observed in extended radio
galaxies, with the potential of removing significant fractions of the ISM from
the host galaxy. The absence of luminous emission line gas extending beyond the
radio emission in these sources agrees with the observed timescales and outflow
rates in extended radio galaxies, and adds further evidence that the energetic,
large-scale outflows observed in extended radio sources (Nesvadba et al. 2006)
are indeed the result of influence of the radio jet.
|
0708.4150v1
|
2008-08-19
|
The Extended Star Formation History of the Andromeda Spheroid at 35 Kpc on the Minor Axis
|
Using the HST ACS, we have obtained deep optical images reaching well below
the oldest main sequence turnoff in fields on the southeast minor-axis of the
Andromeda Galaxy, 35 kpc from the nucleus. These data probe the star formation
history in the extended halo of Andromeda -- that region beyond 30 kpc that
appears both chemically and morphologically distinct from the metal-rich,
highly-disturbed inner spheroid. The present data, together with our previous
data for fields at 11 and 21 kpc, do not show a simple trend toward older ages
and lower metallicities, as one might expect for populations further removed
from the obvious disturbances of the inner spheroid. Specifically, the mean
ages and [Fe/H] values at 11 kpc, 21 kpc, and 35 kpc are 9.7 Gyr and -0.65,
11.0 Gyr and -0.87, and 10.5 Gyr and -0.98, respectively. In the best-fit model
of the 35 kpc population, one third of the stars are younger than 10 Gyr, while
only ~10% of the stars are truly ancient and metal-poor. The extended halo thus
exhibits clear evidence of its hierarchical assembly, and the contribution from
any classical halo formed via early monolithic collapse must be small.
|
0808.2648v1
|
2008-09-30
|
Evidence for powerful AGN winds at high redshift: Dynamics of galactic outflows in radio galaxies during the "Quasar Era"
|
AGN feedback now appears as an attractive mechanism to resolve some of the
outstanding problems with the "standard" cosmological models, in particular
those related to massive galaxies. To directly constrain how this may influence
the formation of massive galaxies near the peak in the redshift distribution of
powerful quasars, z~2, we present an analysis of the emission-line kinematics
of 3 powerful radio galaxies at z~2-3 (HzRGs) based on rest-frame optical
integral-field spectroscopy obtained with SINFONI on the VLT. HzRGs are among
the most massive galaxies, so AGN feedback may have a particularly clear
signature. We find evidence for bipolar outflows in all HzRGs, with kinetic
energies that are equivalent to 0.2% of the rest-mass of the supermassive black
hole. Velocity offsets in the outflows are ~800-1000 km s^-1 between the
blueshifted and redshifted line emission, FWHMs ~1000 km s^-1 suggest strong
turbulence. Ionized gas masses estimated from the Ha luminosity are of order
10^10 M_s, similar to the molecular gas content of HzRGs, underlining that
these outflows may indicate a significant phase in the evolution of the host
galaxy. The total energy release of ~10^60 erg during a dynamical time of ~10^7
yrs corresponds to about the binding energy of a massive galaxy. Geometry,
timescales and energy injection rates of order 10% of the kinetic energy flux
of the jet suggest that the outflows are most likely driven by the radio
source. The global energy density release of ~10^57 erg s^-1 Mpc^-3 may also
influence the subsequent evolution of the HzRG by enhancing the entropy and
pressure in the surrounding halo and facilitating ram-pressure stripping of gas
in satellite galaxies that may contribute to the subsequent mass assembly of
the HzRG through low-dissipation "dry" mergers.
|
0809.5171v2
|
2009-02-16
|
The Role of Dwarf Galaxies in Building Large Stellar Halos
|
The hierarchical theory of galaxy formation rests on the idea that smaller
galactic structures merge to form the galaxies that we see today. The past
decade has provided remarkable observational support for this scenario, driven
in part by advances in spectroscopic instrumentation. Multi-object spectroscopy
enabled the discovery of kinematically cold substructures around the Milky Way
and M31 that are likely the debris of disrupting satellites. Improvements in
high-resolution spectroscopy have produced key evidence that the abundance
patterns of the Milky Way halo and its dwarf satellites can be explained by
Galactic chemical evolution models based on hierarchical assembly.
These breakthroughs have depended almost entirely on observations of nearby
stars in the Milky Way and luminous red giant stars in M31 and Local Group
dwarf satellites. In the next decade, extremely large telescopes will allow
observations far down the luminosity function in the known dwarf galaxies, and
they will enable observations of individual stars far out in the Galactic halo.
The chemical abundance census now available for the Milky Way will become
possible for our nearest neighbor, M31. Velocity dispersion measurements now
available in M31 will become possible for systems beyond the Local Group such
as Sculptor and M81 Group galaxies. Detailed studies of a greater number of
individual stars in a greater number of spiral galaxies and their satellites
will test hierarchical assembly in new ways because dynamical and chemical
evolution models predict different outcomes for halos of different masses in
different environments.
|
0902.2591v2
|
2009-06-12
|
Mass and radius determinations for five transiting M-dwarf stars
|
We have derived masses and radii for both components in five short-period
single-lined eclipsing binary stars discovered by the TrES wide-angle
photometric survey for transiting planets. All these systems consist of a
visible F-star primary and an unseen M-star secondary (M_A > 0.8 M_sun, M_B <
0.45 M_sun). The spectroscopic orbital solution combined with a high precision
transit light curve for each system gives sufficient information to calculate
the density of the primary star and the surface gravity of the secondary. The
masses of the primary stars were obtained using stellar evolution models. The
solutions were compared with results obtained by calculating the radius of the
primary stars under the assumption of rotational synchronization with the
orbital period and alignment between their spin axis and the axis of the orbit.
Four systems show an acceptable match between the two sets of results, but one
system shows a clear mismatch between the two solutions, which may indicate the
absence of synchronization or a misalignment between the rotational and orbital
axis. When compared to low-mass stellar evolution models, the derived masses
and radii of the unseen M dwarfs are inconsistent (three only marginally) with
the predicted values, with all of the radii being larger than expected for
their masses. These results confirm the discrepancy shown in previous work
between the predicted and observed radii on low-mass binary stars. This work
also shows that reliance on the assumption of synchronization to derive the
mass and radius of stars in eclipsing single--lined F+M binaries is a useful
tool, but may not always be warranted and should be carefully tested against
stellar evolution models.
|
0906.2207v2
|
2009-06-23
|
GJ 3236: a new bright, very low-mass eclipsing binary system discovered by the MEarth observatory
|
We report the detection of eclipses in GJ 3236, a bright (I = 11.6) very low
mass binary system with an orbital period of 0.77 days. Analysis of light- and
radial velocity curves of the system yielded component masses of 0.38 +/- 0.02
and 0.28 +/- 0.02 Msol. The central values for the stellar radii are larger
than the theoretical models predict for these masses, in agreement with the
results for existing eclipsing binaries, although the present 5% observational
uncertainties limit the significance of the larger radii to approximately 1
sigma. Degeneracies in the light curve models resulting from the unknown
configuration of surface spots on the components of GJ 3236 currently dominate
the uncertainties in the radii, and could be reduced by obtaining precise,
multi-band photometry covering the full orbital period. The system appears to
be tidally synchronized and shows signs of high activity levels as expected for
such a short orbital period, evidenced by strong Halpha emission lines in the
spectra of both components. These observations probe an important region of
mass-radius parameter space around the predicted transition to fully-convective
stellar interiors, where there are a limited number of precise measurements
available in the literature.
|
0906.4365v1
|
2009-07-03
|
The NGC 300 Transient: An Alternative Method For Measuring Progenitor Masses
|
We present an alternative technique for measuring the precursor masses of
transient events in stars undergoing late stage stellar evolution. We use the
well-established techniques of stellar population modeling to age-date the
stars surrounding the site of the recent transient event in NGC 300 (NGC 300
OT2008-1). The surrounding stars must share a common turnoff mass with the
transient, since almost all stars form in stellar clusters that remain
physically associated for periods longer than the lifetime of the most massive
stars. We find that the precursor of NGC 300 OT2008-1 is surrounded by stars
that formed in a single burst between 8-13 Myr ago, with 70% confidence. The
transient was therefore likely to be due to a progenitor whose mass falls
between the main sequence turnoff mass (12 Msun) and the maximum stellar mass
(25 Msun) found for isochrones bounding this age range. We characterize the
general applicability of this technique in identifying precursor masses of
historic and future transients and supernovae (SNe), noting that it requires
neither precursor imaging nor sub-arcsecond accuracy in the position of the
transient. It is also based on the well-understood physics of the main
sequence, and thus may be a more reliable source of precursor masses than
fitting evolutionary tracks to precursor magnitudes. We speculate that if the
progenitor mass is close to 17 Msun, there may be a connection between optical
transients such as NGC 300 OT2008-1 and the missing type II-P SNe, known as the
"red supergiant problem."
|
0907.0710v2
|
2009-08-03
|
A binary engine fuelling HD87643' s complex circumstellar environment, using AMBER/VLTI
|
Context. The star HD 87643, exhibiting the "B[e] phenomenon", has one of the
most extreme infrared excesses for this object class. It harbours a large
amount of both hot and cold dust, and is surrounded by an extended reflection
nebula. Aims. One of our major goals was to investigate the presence of a
companion in HD87643. In addition, the presence of close dusty material was
tested through a combination of multi-wavelength high spatial 5Aresolution
observations. Methods. We observed HD 87643 with high spatial resolution
techniques, using the near-IR AMBER/VLTI interferometer with baselines ranging
from 60 m to 130 m and the mid-IR MIDI/VLTI interferometer with baselines
ranging from 25 m to 65 m. These observations are complemented by NACO/VLT
adaptive-optics-corrected images in the K and L-bands, ESO-2.2m optical
Wide-Field Imager large-scale images in the B, V and R-bands, Results. We
report the direct detection of a companion to HD 87643 by means of image
synthesis using the AMBER/VLTI instrument. The presence of the companion is
confirmed by the MIDI and NACO data, although with a lower confidence. The
companion is separated by ~ 34 mas with a roughly north-south orientation. The
period must be large (several tens of years) and hence the orbital parameters
are not determined yet. Binarity with high eccentricity might be the key to
interpreting the extreme characteristics of this system, namely a dusty
circumstellar envelope around the primary, a compact dust nebulosity around the
binary system and a complex extended nebula witnessing past violent ejections.
|
0908.0227v2
|
2009-08-12
|
Refined stellar, orbital and planetary parameters of the eccentric HAT-P-2 planetary system
|
We present refined parameters for the extrasolar planetary system HAT-P-2
(also known as HD 147506), based on new radial velocity and photometric data.
HAT-P-2b is a transiting extrasolar planet that exhibits an eccentric orbit. We
present a detailed analysis of the planetary and stellar parameters, yielding
consistent results for the mass and radius of the star, better constraints on
the orbital eccentricity, and refined planetary parameters. The improved
parameters for the host star are M_star = 1.36 +/- 0.04 M_sun and R_star = 1.64
+/- 0.08 R_sun, while the planet has a mass of M_p = 9.09 +/- 0.24 M_Jup and
radius of R_p = 1.16 +/- 0.08 R_Jup. The refined transit epoch and period for
the planet are E = 2,454,387.49375 +/- 0.00074 (BJD) and P = 5.6334729 +/-
0.0000061 (days), and the orbital eccentricity and argument of periastron are e
= 0.5171 +/- 0.0033 and omega = 185.22 +/- 0.95 degrees. These orbital elements
allow us to predict the timings of secondary eclipses with a reasonable
accuracy of ~15 minutes. We also discuss the effects of this significant
eccentricity including the characterization of the asymmetry in the transit
light curve. Simple formulae are presented for the above, and these, in turn,
can be used to constrain the orbital eccentricity using purely photometric
data. These will be particularly useful for very high precision, space-borne
observations of transiting planets.
|
0908.1705v2
|
2009-09-17
|
The SPLASH Survey: A Spectroscopic Analysis of the Metal-Poor, Low-Luminosity M31 dSph Satellite Andromeda X
|
Andromeda X (And X) is a newly discovered low-luminosity M31 dwarf spheroidal
galaxy (dSph) found by Zucker et al. (2007) in the Sloan Digital Sky Survey
(SDSS - York et al. 2000). In this paper, we present the first spectroscopic
study of individual red giant branch stars in And X, as a part of the SPLASH
Survey (Spectroscopic and Photometric Landscape of Andromeda's Stellar Halo).
Using the Keck II telescope and multiobject DEIMOS spectrograph, we target two
spectroscopic masks over the face of the galaxy and measure radial velocities
for ~100 stars with a median accuracy of sigma_v ~ 3 km/s. The velocity
histogram for this field confirms three populations of stars along the sight
line: foreground Milky Way dwarfs at small negative velocities, M31 halo red
giants over a broad range of velocities, and a very cold velocity ``spike''
consisting of 22 stars belonging to And X with v_rad = -163.8 +/- 1.2 km/s. By
carefully considering both the random and systematic velocity errors of these
stars (e.g., through duplicate star measurements), we derive an intrinsic
velocity dispersion of just sigma_v = 3.9 +/- 1.2 km/s for And X, which for its
size, implies a minimum mass-to-light ratio of M/L =37^{+26}_{-19} assuming the
mass traces the light. Based on the clean sample of member stars, we measure
the median metallicity of And X to be [Fe/H] = -1.93 +/- 0.11, with a slight
radial metallicity gradient. The dispersion in metallicity is large,
sigma([Fe/H]) = 0.48, possibly hinting that the galaxy retained much of its
chemical enrichment products. We discuss the potential for better understanding
the formation and evolution mechanisms for M31's system of dSphs through
(current) kinematic and chemical abundance studies, especially in relation to
the Milky Way sample. (abridged version)
|
0909.3298v1
|
2009-11-11
|
The SPLASH Survey: Internal Kinematics, Chemical Abundances, and Masses of the Andromeda I, II, III, VII, X, and XIV dSphs
|
We present new Keck/DEIMOS spectroscopic observations of hundreds of
individual stars along the sightline to Andromeda's first three discovered
dwarf spheroidal galaxies (dSphs) - And I, II, and III, and leverage recent
observations by our team of three additional dSphs, And VII, X, and XIV, as a
part of the SPLASH Survey. Member stars of each dSph are isolated from
foreground Milky Way dwarf and M31 field contamination using a variety of
photometric and spectroscopic diagnostics. Our final spectroscopic sample of
member stars in each dSph, for which we measure accurate radial velocities with
a median uncertainty (random plus systematic errors) of 4 - 5 km/s, includes 80
red giants in And I, 95 in And II, 43 in And III, 18 in And VII, 22 in And X,
and 38 in And XIV. The sample of confirmed members in the six dSphs are used to
derive each system's mean radial velocity, intrinsic central velocity
dispersion, mean abundance, abundance spread, and dynamical mass. This combined
data set presents us with a unique opportunity to perform the first systematic
comparison of the global properties (e.g., metallicities, sizes, and dark
matter masses) of one-third of Andromeda's total known dSph population with
Milky Way counterparts of the same luminosity. We discuss both the
luminosity-metallicity relation and the luminosity-size relation of these
satellites, and find that the chemical evolution histories of each host's
satellites is similar. The dynamical mass estimates of M31's dSphs are similar
or smaller than Milky Way dSphs of the same luminosity despite their sizes
being similar or larger, suggesting M31 dSphs are less dense than Milky Way
counterparts. The implications of these results for general understanding of
galaxy formation and evolution is summarized. Abridged.
|
0911.1998v3
|
2009-11-19
|
Local Group Dwarf Elliptical Galaxies: II. Stellar Kinematics to Large Radii in NGC 147 and NGC 185
|
We present kinematic and metallicity profiles for the M31 dwarf elliptical
(dE) satellite galaxies NGC 147 and NGC 185. The profiles represent the most
extensive spectroscopic radial coverage for any dE galaxy, extending to a
projected distance of eight half-light radii (8 r_eff = 14'). We achieve this
coverage via Keck/DEIMOS multislit spectroscopic observations of 520 and 442
member red giant branch stars in NGC 147 and NGC 185, respectively. In contrast
to previous studies, we find that both dEs have significant internal rotation.
We measure a maximum rotational velocity of 17+/-2 km/s for NGC 147 and 15+/-5
km/s for NGC 185. The velocity dispersions decrease gently with radius with an
average dispersion of 16+/-1 km/s for NGC 147 and 24+/-1 km/s for NGC 185. Both
dEs have internal metallicity dispersions of 0.5 dex, but show no evidence for
a radial metallicity gradient. We construct two-integral axisymmetric dynamical
models and find that the observed kinematical profiles cannot be explained
without modest amounts of non-baryonic dark matter. We measure central
mass-to-light ratios of ML_V = 4.2+/-0.6 and ML_V = 4.6+/-0.6 for NGC 147 and
NGC 185, respectively. Both dE galaxies are consistent with being primarily
flattened by their rotational motions, although some anisotropic velocity
dispersion is needed to fully explain their observed shapes. The velocity
profiles of all three Local Group dEs (NGC 147, NGC 185 and NGC 205) suggest
that rotation is more prevalent in the dE galaxy class than previously assumed,
but is often manifest only at several times the effective radius. Since all dEs
outside the Local Group have been probed to only inside the effective radius,
this opens the door for formation mechanisms in which dEs are transformed or
stripped versions of gas-rich rotating progenitor galaxies.
|
0911.3654v2
|
2009-11-20
|
The ACS Nearby Galaxy Survey Treasury IV. The Star Formation History of NGC 2976
|
We present resolved stellar photometry of NGC 2976 obtained with the Advanced
Camera for Surveys (ACS) as part of the ACS Nearby Galaxy Survey Treasury
(ANGST) program. The data cover the radial extent of the major axis of the disk
out to 6 kpc, or ~6 scale lengths. The outer disk was imaged to a depth of
M_F606W ~ 1, and an inner field was imaged to the crowding limit at a depth of
M_F606W ~ -1. Through detailed analysis and modeling of these CMDs we have
reconstructed the star formation history of the stellar populations currently
residing in these portions of the galaxy, finding similar ancient populations
at all radii but significantly different young populations at increasing radii.
In particular, outside of the well-measured break in the disk surface
brightness profile, the age of the youngest population increases with distance
from the galaxy center, suggesting that star formation is shutting down from
the outside-in. We use our measured star formation history, along with H I
surface density measurements, to reconstruct the surface density profile of the
disk during previous epochs. Comparisons between the recovered star formation
rates and reconstructed gas densities at previous epochs are consistent with
star formation following the Schmidt law during the past 0.5 Gyrs, but with a
drop in star formation efficiency at low gas densities, as seen in local
galaxies at the present day. The current rate and gas density suggest that
rapid star formation in NGC 2976 is currently in the process of ceasing from
the outside-in due to gas depletion. This process of outer disk gas depletion
and inner disk star formation was likely triggered by an interaction with the
core of the M81 group >~1 Gyr ago that stripped the gas from the galaxy halo
and/or triggered gas inflow from the outer disk toward the galaxy center.
|
0911.4121v1
|
2009-12-07
|
OB Stars & Stellar Bowshocks in Cygnus-X: A Novel Laboratory Estimating Stellar Mass Loss Rates
|
We use mid-IR images from the Spitzer Cygnus~X Legacy Survey to search for
stellar bowshocks, a signature of early type "runaway" stars with high space
velocities. We identify ten arc-shaped nebulae containing centrally located
stars as candidate bowshocks. New spectroscopic observations of five stars show
that all are late O to early B dwarfs. Our morphologically selected sample of
bowshock candidates encompasses diverse physical phenomena. Three of the stars
appear to be pre-main-sequence objects on the basis of rising SEDs in the
mid-IR, and their nebulae may be photon-dominated regions (PDRs). Four objects
have ambiguous classification. These may be partial dust shells or bubbles. We
conclude that three of the objects are probable bowshocks, based on their
morphological similarity to analytic prescriptions. Their nebular morphologies
reveal no systematic pattern of orientations that might indicate either a
population of stars ejected from or large-scale hydrodynamic outflows from Cyg
OB2. The fraction of runaways among OB stars near Cyg OB2 identified either by
radial velocity or bowshock techniques is ~0.5%, much smaller than the 8%
estimated among field OB stars. We also obtained a heliocentric radial velocity
for the previously known bowshock star, BD+43\degr3654, of -66.2+/-9.4 km/s,
solidifying its runaway status and implying a space velocity of 77+/-10 km/s.
We use the principles of momentum-driven bowshocks to arrive at a novel method
for estimating stellar mass loss rates. Derived mass loss rates range between
10^-7 and few x10^-6 solar masses/yr for the three O5V -- ~B2V stars identified
as generating bowshocks. These values are at the upper range of, but broadly
consistent with, estimates from other methods. (Abridged)
|
0912.1314v1
|
2010-01-27
|
Mass-radius relationships for exoplanets
|
For planets other than Earth, interpretation of the composition and structure
depends largely on comparing the mass and radius with the composition expected
given their distance from the parent star. The composition implies a
mass-radius relation which relies heavily on equations of state calculated from
electronic structure theory and measured experimentally on Earth. We lay out a
method for deriving and testing equations of state, and deduce mass-radius and
mass-pressure relations for key materials whose equation of state is reasonably
well established, and for differentiated Fe/rock. We find that variations in
the equation of state, such as may arise when extrapolating from low pressure
data, can have significant effects on predicted mass- radius relations, and on
planetary pressure profiles. The relations are compared with the observed
masses and radii of planets and exoplanets. Kepler-10b is apparently 'Earth-
like,' likely with a proportionately larger core than Earth's, nominally 2/3 of
the mass of the planet. CoRoT-7b is consistent with a rocky mantle over an
Fe-based core which is likely to be proportionately smaller than Earth's. GJ
1214b lies between the mass-radius curves for H2O and CH4, suggesting an 'icy'
composition with a relatively large core or a relatively large proportion of
H2O. CoRoT-2b is less dense than the hydrogen relation, which could be
explained by an anomalously high degree of heating or by higher than assumed
atmospheric opacity. HAT-P-2b is slightly denser than the mass-radius relation
for hydrogen, suggesting the presence of a significant amount of matter of
higher atomic number. CoRoT-3b lies close to the hydrogen relation. The
pressure at the center of Kepler-10b is 1.5+1.2-1.0 TPa. The central pressure
in CoRoT-7b is probably close to 0.8TPa, though may be up to 2TPa.
|
1001.4851v3
|
2010-02-09
|
The ACS Nearby Galaxy Survey Treasury V. Radial Star Formation History of NGC 300
|
We present new Hubble Space Telescope (HST) observations of NGC 300 taken as
part of the ACS Nearby Galaxy Survey Treasury (ANGST). Individual stars are
resolved in these images down to an absolute magnitude of M_F814W = 1.0 (below
the red clump). We determine the star formation history of the galaxy in 6
radial bins by comparing our observed color-magnitude diagrams (CMDs) with
synthetic CMDs based on theoretical isochrones. We find that the stellar disk
out to 5.4 kpc is primarily old, in contrast with the outwardly similar galaxy
M33. We determine the scale length as a function of age and find evidence for
inside-out growth of the stellar disk: the scale length has increased from 1.1
+/- 0.1 kpc 10 Gyr ago to 1.3 +/- 0.1 kpc at present, indicating a buildup in
the fraction of young stars at larger radii. As the scale length of M33 has
recently been shown to have increased much more dramatically with time, our
results demonstrate that two galaxies with similar sizes and morphologies can
have very different histories. With an N-body simulation of a galaxy designed
to be similar to NGC 300, we determine that the effects of radial migration
should be minimal. We trace the metallicity gradient as a function of time and
find a present day metallicity gradient consistent with that seen in previous
studies. Consistent results are obtained from archival images covering the same
radial extent but differing in placement and filter combination.
|
1002.1743v1
|
2010-04-28
|
The ACS Nearby Galaxy Survey Treasury VI. The Ancient Star Forming disk of NGC 404
|
We present HST/WFPC2 observations across the disk of the nearby isolated
dwarf S0 galaxy NGC 404, which hosts an extended gas disk. Our deepest field
reaches the red clump and main-sequence stars with ages <500 Myr. Although we
detect trace amounts of star formation at times more recent than 10 Gyr for all
fields, the proportion of red giant stars to asymptotic giants and main
sequence stars suggests that the disk is dominated by an ancient (>10 Gyr)
population. Detailed modeling of the color-magnitude diagram suggests that ~70%
of the stellar mass in the NGC 404 disk formed by z~2 (10 Gyr ago) and at least
~90% formed prior to z~1 (8 Gyr ago). These results indicate that the stellar
populations of the NGC 404 disk are on average significantly older than those
of other nearby disk galaxies, suggesting that early and late type disks may
have different long-term evolutionary histories, not simply differences in
their recent star formation rates. Comparisons of the spatial distribution of
the young stellar mass and FUV emission in GALEX images show that the brightest
FUV regions contain the youngest stars, but that some young stars (<160 Myr)
lie outside of these regions. FUV luminosity appears to be strongly affected by
both age and stellar mass within individual regions. Finally, we use our
measurements to infer the relationship between the star formation rate and the
gas density of the disk at previous epochs. We find that most of the history of
the NGC 404 disk is consistent with star formation that has decreased with the
gas density according to the Schmidt law. However, 0.5-1 Gyr ago, the star
formation rate was unusually low for the inferred gas density, consistent with
the possibility that there was a gas accretion event that reignited star
formation ~0.5 Gyr ago. Such an event could explain why this S0 galaxy hosts an
extended gas disk.
|
1004.5135v1
|
2010-06-13
|
The 21 cm Signature of Cosmic String Wakes
|
We discuss the signature of a cosmic string wake in 21cm redshift surveys.
Since 21cm surveys probe higher redshifts than optical large-scale structure
surveys, the signatures of cosmic strings are more manifest in 21cm maps than
they are in optical galaxy surveys. We find that, provided the tension of the
cosmic string exceeds a critical value (which depends on both the redshift when
the string wake is created and the redshift of observation), a cosmic string
wake will generate an emission signal with a brightness temperature which
approaches a limiting value which at a redshift of $z + 1 = 30$ is close to 400
mK in the limit of large string tension. The signal will have a specific
signature in position space: the excess 21cm radiation will be confined to a
wedge-shaped region whose tip corresponds to the position of the string, whose
planar dimensions are set by the planar dimensions of the string wake, and
whose thickness (in redshift direction) depends on the string tension. For
wakes created at $z_i + 1 = 10^3$, then at a redshift of $z + 1 = 30$ the
critical value of the string tension $\mu$ is $G \mu = 6 \times 10^{-7}$, and
it decreases linearly with redshift (for wakes created at the time of equal
matter and radiation, the critical value is a factor of two lower at the same
redshift). For smaller tensions, cosmic strings lead to an observable
absorption signal with the same wedge geometry.
|
1006.2514v3
|
2010-09-23
|
The ACS Nearby Galaxy Survey Treasury IX. Constraining asymptotic giant branch evolution with old metal-poor galaxies
|
In an attempt to constrain evolutionary models of the asymptotic giant branch
(AGB) phase at the limit of low masses and low metallicities, we have examined
the luminosity functions and number ratio between AGB and red giant branch
(RGB) stars from a sample of resolved galaxies from the ACS Nearby Galaxy
Survey Treasury (ANGST). This database provides HST optical photometry together
with maps of completeness, photometric errors, and star formation histories for
dozens of galaxies within 4 Mpc. We select 12 galaxies characterized by
predominantly metal-poor populations as indicated by a very steep and blue RGB,
and which do not present any indication of recent star formation in their
color--magnitude diagrams. Thousands of AGB stars brighter than the tip of the
RGB (TRGB) are present in the sample (between 60 and 400 per galaxy), hence the
Poisson noise has little impact in our measurements of the AGB/RGB ratio. We
model the photometric data with a few sets of thermally pulsing AGB (TP-AGB)
evolutionary models with different prescriptions for the mass loss. This
technique allows us to set stringent constraints to the TP-AGB models of
low-mass metal-poor stars (with M<1.5 Msun, [Fe/H]<~-1.0). Indeed, those which
satisfactorily reproduce the observed AGB/RGB ratios have TP-AGB lifetimes
between 1.2 and 1.8 Myr, and finish their nuclear burning lives with masses
between 0.51 and 0.55 Msun. This is also in good agreement with recent
observations of white dwarf masses in the M4 old globular cluster. These
constraints can be added to those already derived from Magellanic Cloud star
clusters as important mileposts in the arduous process of calibrating AGB
evolutionary models.
|
1009.4618v1
|
2010-11-08
|
Least Squares Ranking on Graphs
|
Given a set of alternatives to be ranked, and some pairwise comparison data,
ranking is a least squares computation on a graph. The vertices are the
alternatives, and the edge values comprise the comparison data. The basic idea
is very simple and old: come up with values on vertices such that their
differences match the given edge data. Since an exact match will usually be
impossible, one settles for matching in a least squares sense. This formulation
was first described by Leake in 1976 for rankingfootball teams and appears as
an example in Professor Gilbert Strang's classic linear algebra textbook. If
one is willing to look into the residual a little further, then the problem
really comes alive, as shown effectively by the remarkable recent paper of
Jiang et al. With or without this twist, the humble least squares problem on
graphs has far-reaching connections with many current areas ofresearch. These
connections are to theoretical computer science (spectral graph theory, and
multilevel methods for graph Laplacian systems); numerical analysis (algebraic
multigrid, and finite element exterior calculus); other mathematics (Hodge
decomposition, and random clique complexes); and applications (arbitrage, and
ranking of sports teams). Not all of these connections are explored in this
paper, but many are. The underlying ideas are easy to explain, requiring only
the four fundamental subspaces from elementary linear algebra. One of our aims
is to explain these basic ideas and connections, to get researchers in many
fields interested in this topic. Another aim is to use our numerical
experiments for guidance on selecting methods and exposing the need for further
development.
|
1011.1716v4
|
2010-12-20
|
Integrable embeddings and foliations
|
A k-submanifold L of an open n-manifold M is called weakly integrable (WI)
[resp. strongly integrable (SI)] if there exists a submersion \Phi:M\to R^{n-k}
such that L\subset \Phi^{-1}(0) [resp. L= \Phi^{-1}(0)]. In this work we study
the following problem, first stated in a particular case by Costa et al.
(Invent. Math. 1988): which submanifolds L of an open manifold M are WI or SI?
For general M, we explicitly solve the case k=n-1 and provide necessary and
sufficient conditions for submanifolds to be WI and SI in higher codimension.
As particular cases we recover the theorem of Bouma and Hector (Indagationes
Math. 1983) asserting that any open orientable surface is SI in R^3, and
Watanabe's and Miyoshi's theorems (Topology 1993 and 1995) claiming that any
link is WI in an open 3-manifold. In the case M=R^n we fully characterize WI
and SI submanifolds, we provide examples of 3- and 7-manifolds which are not WI
and we show that a theorem by Miyoshi (Topology 1995) which states that any
link in R^3 is SI does not hold in general. The right analogue to Miyoshi's
theorem is also proved, implying in particular the surprising result that no
knot in R^3 is SI. Our results applied to the theory of foliations of Euclidean
spaces give rise to some striking corollaries: using some topological
invariants we classify all the submanifolds of R^n which can be realized as
proper leaves of foliations; we prove that S^3 can be realized as a leaf of a
foliation of R^n, n\geq 7, but not in R^5 or R^6, which partially answers a
question by Vogt (Math. Ann. 1993); we construct open 3-manifolds which cannot
be leaves of a foliation of any compact 4-manifold but are proper leaves in
R^4. The theory of WI and SI submanifolds is a framework where many classical
tools of differential and algebraic topology play a prominent role:
h-principle, complete intersections and the theory of immersions and
embeddings.
|
1012.4312v1
|
2011-01-06
|
How Typical Are The Local Group Dwarf Galaxies?
|
We compare the cumulative star formation histories (SFHs) of Local Group (LG)
dwarf galaxies with those in the volume-limited ACS Nearby Galaxy Survey
Treasury (ANGST) sample (D < 4 Mpc), in order to understand how typical the LG
dwarf galaxies are relative to those in the nearby universe. The SFHs were
derived in a uniform manner from high quality optical color-magnitude diagrams
constructed from Hubble Space Telescope imaging. We find that the {\it mean}
cumulative SFHs of the LG dwarfs are comparable to the mean cumulative SFHs of
the ANGST sample for the three different morphological types (dwarf
spheroidals/ellipticals: dSph/dE; dwarf irregulars: dI; transition dwarfs:
dTrans). We also discuss effects such as population gradients and systematic
uncertainties in the stellar models that may influence the derived SFHs. Both
the ANGST and Local Group dwarf galaxies show a consistent and strong
morphology-density relationship, emphasizing the importance of environment in
the evolution of dwarf galaxies. Specifically, we confirm that dIs are found at
lower densities and higher luminosities than dSphs, within this large sample.
We also find that dTrans are located in similar environments to those occupied
by dwarf irregular galaxies, but have systematically lower luminosities that
are more comparable to those of dwarf spheroidals. The similarity of the SFHs
and morphology-density relationships of the LG and ANGST dwarf galaxies
suggests that the LG dwarfs are a good representation of dwarf galaxies in the
local universe.
|
1101.1301v2
|
2011-01-18
|
HAT-P-27b: A hot Jupiter transiting a G star on a 3 day orbit
|
We report the discovery of HAT-P-27b, an exoplanet transiting the moderately
bright G8 dwarf star GSC 0333-00351 (V=12.214). The orbital period is 3.039586
+/- 0.000012 d, the reference epoch of transit is 2455186.01879 +/- 0.00054
(BJD), and the transit duration is 0.0705 +/- 0.0019 d. The host star with its
effective temperature 5300 +/- 90 K is somewhat cooler than the Sun, and is
more metal-rich with a metallicity of +0.29 +/- 0.10. Its mass is 0.94 +/- 0.04
Msun and radius is 0.90 +/- 0.04 Rsun. For the planetary companion we determine
a mass of 0.660 +/- 0.033 MJ and radius of 1.038 +0.077 -0.058 RJ. For the 30
known transiting exoplanets between 0.3 MJ and 0.8 MJ, a negative correlation
between host star metallicity and planetary radius, and an additional
dependence of planetary radius on equilibrium temperature are confirmed at a
high level of statistical significance.
|
1101.3511v2
|
2011-03-31
|
The ACS Nearby Galaxy Survey Treasury VII. The NGC 4214 Starburst and the Effects of Star Formation History on Dwarf Morphology
|
We present deep Hubble Space Telescope Wide Field Planetary Camera 2 (WFPC2)
optical observations obtained as part of the ACS Nearby Galaxy Survey Treasury
(ANGST) as well as early release Wide Field Camera 3 (WFC3) ultra-violet and
infrared observations of the nearby dwarf starbursting galaxy NGC 4214. Our
data provide a detailed example of how covering such a broad range in
wavelength provides a powerful tool for constraining the physical properties of
stellar populations. The deepest data reach the ancient red clump at M_F814W
-0.2. All of the optical data reach the main sequence turnoff for stars younger
than ~300 Myr, and the blue He burning sequence for stars younger than 500 Myr.
The full CMD-fitting analysis shows that all three fields in our data set are
consistent with ~75% of the stellar mass being older than 8 Gyr, in spite of
showing a wide range in star formation rates at the present day. Thus, our
results suggest that the scale length of NGC 4214 has remained relatively
constant for many Gyr. As previously noted by others, we also find the galaxy
has recently ramped up production, consistent with its bright UV luminosity and
its population of UV-bright massive stars. In the central field we find UV
point sources with F336W magnitudes as bright as -9.9. These are as bright as
stars with masses of at least 52-56 M_sun and ages near 4 Myr in stellar
evolution models. Assuming a standard IMF, our CMD is well-fitted by an
increase in star formation rate beginning 100 Myr ago. The stellar populations
of this late-type dwarf are compared with those of NGC 404, an early-type dwarf
that is also the most massive galaxy in its local environment. The late-type
dwarf appears to have a similar high fraction of ancient stars, suggesting that
these dominant galaxies may form at early epochs even if they have low total
mass and very different present-day morphologies.
|
1104.0033v1
|
2011-05-10
|
The Higgs Potential in the Type II Seesaw Model
|
We perform in the type II seesaw setting, a detailed study of the dynamical
features of the corresponding general renormalizable doublet/triplet Higgs
potential that depends on five dimensionless couplings and two mass parameters
after spontaneous symmetry breaking, and highlight the implications for the
Higgs phenomenology. In particular, we determine i) the complete set of
tree-level unitarity constraints on the couplings of the potential and ii) the
exact tree-level {\sl all directions} boundedness from below constraints on
these couplings. When combined, these constraints delineate precisely the
theoretically allowed parameter space domain within our perturbative
approximation. Among the seven physical Higgs states of this model, the mass of
the lighter (heavier) CP-even state h0 (H0) will always satisfy a theoretical
upper (lower) bound that is reached for a critical value mu_c of mu (the mass
parameter controlling triple couplings among the doublet/triplet Higgses).
Saturating the unitarity bounds we find m_h0 < {\cal O}(0.7 - 1 TeV), while the
upper bound for the remaining Higgses lies in the several tens of TeV. However,
the actual masses can be much lighter. We identify two regimes corresponding to
mu > mu_c and mu < mu_c. In the first regime the Higgs sector is typically very
heavy and only h0 that becomes SM-like could be accessible to the LHC. In
contrast, in the second regime, somewhat overlooked in the literature, most of
the Higgs sector is light. In particular the heaviest state H0 becomes SM-like,
the lighter states being the CP-odd Higgs, the (doubly) charged Higgses and a
decoupled h0, possibly leading to a distinctive phenomenology at the colliders.
|
1105.1925v2
|
2011-06-17
|
The Luminosity Profile and Structural Parameters of the Andromeda Galaxy
|
We have constructed an extended composite luminosity profile for the
Andromeda galaxy, M31, and have decomposed it into three basic luminous
structural components: a bulge, a disk and a halo. The dust-free Spitzer/IRAC
imaging and extended spatial coverage of ground-based optical imaging and deep
star counts allow us to map M31's structure from its center to 22 kpc along the
major axis. We apply different decomposition methods for the 1D luminosity
profiles and 2D images. These include non-linear least-squares and Bayesian
Monte-Carlo Markov-chain analyses. The basic photometric model for M31 has a
Sersic bulge with shape index n = 2.2 +/- 0.3 and effective radius R_e = 1.0
+/- 0.2 kpc, a dust-free exponential disk of scale length R_d = 5.3 +/- 0.5
kpc; the parameter errors reflect the range between various decomposition
methods. The bulge parameter, n, is rather insensitive to bandpass effects and
its value suggests a first rapid formation via mergers followed by secular
growth from the disk. The M31 halo has a 2D power-law index ~ -2.5 +/- 0.2. We
find that the M31 bulge light is mostly dominant over the range R_minor < 1.2
kpc. The disk takes over in the range 1.2 kpc < R_minor < 9 kpc, whereas the
halo dominates at R_minor > 9 kpc. The stellar nucleus, bulge, disk, and halo
components each contribute roughly 0.05%, 23%, 73% and 4% of the total light of
M31 out to 200 kpc along the minor axis. Nominal errors for the structural
parameters of the M31 bulge, disk and halo amount to 20%. If M31 and the Milky
Way are at all typical, faint stellar halos should be routinely detected in
galaxy surveys reaching below mu_i ~ 27 mag/arcsec^2. We stress that our
results rely on this photometric analysis alone. Structural parameters may
change when other fundamental constraints, such as those provided by abundance
gradients and stellar kinematics, are considered simultaneously (Abridged).
|
1106.3564v1
|
2011-08-24
|
The Canada-France Ecliptic Plane Survey - Full Data Release: The orbital structure of the Kuiper belt
|
We report the orbital distribution of the trans-neptunian objects (TNOs)
discovered during the Canada-France Ecliptic Plane Survey, whose discovery
phase ran from early 2003 until early 2007. The follow-up observations started
just after the first discoveries and extended until late 2009. We obtained
characterized observations of 321 sq.deg. of sky to depths in the range g ~
23.5--24.4 AB mag. We provide a database of 169 TNOs with high-precision
dynamical classification and known discovery efficiency. Using this database,
we find that the classical belt is a complex region with sub-structures that go
beyond the usual splitting of inner (interior to 3:2 mean-motion resonance
[MMR]), outer (exterior to 2:1 MMR), and main (in between). The main classical
belt (a=40--47 AU) needs to be modeled with at least three components: the
`hot' component with a wide inclination distribution and two `cold' components
(stirred and kernel) with much narrower inclination distributions. The hot
component must have a significantly shallower absolute magnitude (Hg)
distribution than the other two components. With 95% confidence, there are
8000+1800-1600 objects in the main belt with Hg <= 8.0, of which 50% are from
the hot component, 40% from the stirred component and 10% from the kernel; the
hot component's fraction drops rapidly with increasing Hg. Because of this, the
apparent population fractions depend on the depth and ecliptic latitude of a
trans-neptunian survey. The stirred and kernel components are limited to only a
portion of the main belt, while we find that the hot component is consistent
with a smooth extension throughout the inner, main and outer regions of the
classical belt; the inner and outer belts are consistent with containing only
hot-component objects. The Hg <= 8.0 TNO population estimates are 400 for the
inner belt and 10,000 for the outer belt within a factor of two.
|
1108.4836v1
|
2011-09-09
|
LSPM J1112+7626: detection of a 41-day M-dwarf eclipsing binary from the MEarth transit survey
|
We report the detection of eclipses in LSPM J1112+7626, which we find to be a
moderately bright (I_C = 12.14 +/- 0.05) very low-mass binary system with an
orbital period of 41.03236 +/- 0.00002 days, and component masses M_1 = 0.395
+/- 0.002 Msol and M_2 = 0.275 +/- 0.001 Msol in an eccentric (e = 0.239 +/-
0.002) orbit. A 65 day out of eclipse modulation of approximately 2%
peak-to-peak amplitude is seen in I-band, which is probably due to rotational
modulation of photospheric spots on one of the binary components. This paper
presents the discovery and characterization of the object, including radial
velocities sufficient to determine both component masses to better than 1%
precision, and a photometric solution. We find that the sum of the component
radii, which is much better-determined than the individual radii, is inflated
by 3.8 +0.9 -0.5 % compared to the theoretical model predictions, depending on
the age and metallicity assumed. These results demonstrate that the
difficulties in reproducing observed M-dwarf eclipsing binary radii with
theoretical models are not confined to systems with very short orbital periods.
This object promises to be a fruitful testing ground for the hypothesized link
between inflated radii in M-dwarfs and activity.
|
1109.2055v1
|
2011-09-15
|
Kepler-16: A Transiting Circumbinary Planet
|
We report the detection of a planet whose orbit surrounds a pair of low-mass
stars. Data from the Kepler spacecraft reveal transits of the planet across
both stars, in addition to the mutual eclipses of the stars, giving precise
constraints on the absolute dimensions of all three bodies. The planet is
comparable to Saturn in mass and size, and is on a nearly circular 229-day
orbit around its two parent stars. The eclipsing stars are 20% and 69% as
massive as the sun, and have an eccentric 41-day orbit. The motions of all
three bodies are confined to within 0.5 degree of a single plane, suggesting
that the planet formed within a circumbinary disk.
|
1109.3432v1
|
2011-09-30
|
Resolved Near-Infrared Stellar Populations in Nearby Galaxies
|
We present near-infrared (NIR) color-magnitude diagrams (CMDs) for the
resolved stellar populations within 26 fields of 23 nearby galaxies (<4 Mpc),
based on F110W and F160W images from Wide Field Camera 3 (WFC3) on the Hubble
Space Telescope (HST). The CMDs sample both old dormant and young star-forming
populations. We match key NIR CMD features with their counterparts in optical
CMDs, and identify the red core Helium burning (RHeB) sequence as a significant
contributor to the NIR flux in stellar populations younger than a few 100 Myrs
old, suggesting that star formation can drive surprisingly rapid variations in
the NIR mass-to-light ratio. The NIR luminosity of star forming galaxies is
therefore not necessarily proportional to the stellar mass. We note that these
individual bright RHeB stars may be misidentified as old stellar clusters in
low resolution imaging. We also discuss the CMD location of asymptotic giant
branch (AGB) stars, and the separation of AGB sub-populations using a
combination of optical and NIR colors. We empirically calibrate the NIR
magnitude of the tip of the red giant branch (TRGB) as a function of color,
allowing this widely adopted filter to be used for distance measurements. We
find a clear trend between NIR RGB color and metallicity. However, it appears
unlikely that the slope of the NIR RGB can be used as a metallicity indicator
in extragalactic systems with comparable data. Finally, we discuss scattered
light in the WFC3, which becomes significant for exposures taken close to a
bright earth limb.
|
1109.6893v1
|
2011-10-13
|
The First Stray Light Corrected EUV Images of Solar Coronal Holes
|
Coronal holes are the source regions of the fast solar wind, which fills most
of the solar system volume near the cycle minimum. Removing stray light from
extreme ultraviolet (EUV) images of the Sun's corona is of high astrophysical
importance, as it is required to make meaningful determinations of temperatures
and densities of coronal holes. EUV images tend to be dominated by the
component of the stray light due to the long-range scatter caused by
microroughness of telescope mirror surfaces, and this component has proven very
difficult to measure in pre-flight characterization. In-flight characterization
heretofore has proven elusive due to the fact that the detected image is
simultaneously nonlinear in two unknown functions: the stray light pattern and
the true image which would be seen by an ideal telescope. Using a constrained
blind deconvolution technique that takes advantage of known zeros in the true
image provided by a fortuitous lunar transit, we have removed the stray light
from solar images seen by the EUVI instrument on STEREO-B in all four filter
bands (171, 195, 284, and 304 \AA). Uncertainty measures of the stray light
corrected images, which include the systematic error due to misestimation of
the scatter, are provided. It is shown that in EUVI, stray light contributes up
to 70% of the emission in coronal holes seen on the solar disk, which has
dramatic consequences for diagnostics of temperature and density and therefore
estimates of key plasma parameters such as the plasma $\beta$\ and ion-electron
collision rates.
|
1110.3052v2
|
2011-10-26
|
Qatar-2: A K dwarf orbited by a transiting hot Jupiter and a more massive companion in an outer orbit
|
We report the discovery and initial characterization of Qatar-2b, a hot
Jupiter transiting a V = 13.3 mag K dwarf in a circular orbit with a short
period, P_ b = 1.34 days. The mass and radius of Qatar-2b are M_p = 2.49 M_j
and R_p = 1.14 R_j, respectively. Radial-velocity monitoring of Qatar-2 over a
span of 153 days revealed the presence of a second companion in an outer orbit.
The Systemic Console yielded plausible orbits for the outer companion, with
periods on the order of a year and a companion mass of at least several M_j.
Thus Qatar-2 joins the short but growing list of systems with a transiting hot
Jupiter and an outer companion with a much longer period. This system
architecture is in sharp contrast to that found by Kepler for multi-transiting
systems, which are dominated by objects smaller than Neptune, usually with
tightly spaced orbits that must be nearly coplanar.
|
1110.5912v1
|
2011-11-01
|
Cosmological evolution of warm dark matter fluctuations I: Efficient computational framework with Volterra integral equations
|
We study the complete cosmological evolution of dark matter (DM) density
fluctuations for DM particles that decoupled being ultrarelativistic during the
radiation dominated era which is the case of keV scale warm DM (WDM). The new
framework presented here can be applied to other types of DM and in particular
we extend it to cold DM (CDM). The collisionless and linearized
Boltzmann-Vlasov equations (B-V) for WDM and neutrinos in the presence of
photons and coupled to the linearized Einstein equations are studied in detail
in the presence of anisotropic stress with the Newtonian potential generically
different from the spatial curvature perturbations. We recast this full system
of B-V equations for DM and neutrinos into a system of coupled Volterra
integral equations. These Volterra-type equations are valid both in the
radiation dominated (RD) and matter dominated (MD) eras during which the WDM
particles are ultrarelativistic and then nonrelativistic. This generalizes the
so-called Gilbert integral equation only valid for nonrelativistic particles in
the MD era. We succeed to reduce the system of four Volterra integral equations
for the density and anisotropic stress fluctuations of DM and neutrinos into a
system of only two coupled Volterra equations. The kernels and inhomogeneities
in these equations are explicitly given functions. Combining the
Boltzmann-Vlasov equations and the linearized Einstein equations constrain the
initial conditions on the distribution functions and gravitational potentials.
In the absence of neutrinos the anisotropic stress vanishes and the
Volterra-type equations reduce to a single integral equation. These Volterra
integral equations provide a useful and precise framework to compute the
primordial WDM fluctuations over a wide range of scales including small scales
up to k ~ 1/5 kpc.
|
1111.0290v2
|
2011-12-06
|
Towards resolution of the Fermi surface in underdoped high-Tc superconductors
|
We survey recent experimental results including quantum oscillations and
complementary measurements probing the electronic structure of underdoped
cuprates, and theoretical proposals to explain them. We discuss quantum
oscillations measured at high magnetic fields in the underdoped cuprates that
reveal a small Fermi surface section comprising quasiparticles that obey
Fermi-Dirac statistics, unaccompanied by other states of comparable
thermodynamic mass at the Fermi level. The location of the observed Fermi
surface section at the nodes is indicated by a body of evidence including the
collapse in Fermi velocity measured by quantum oscillations, which is found to
be associated with the nodal density of states observed in angular resolved
photoemission, the persistence of quantum oscillations down to low fields in
the vortex state, the small value of density of states from heat capacity and
the multiple frequency quantum oscillation pattern consistent with nodal
magnetic breakdown of bilayer-split pockets. A nodal Fermi surface pocket is
further consistent with the observation of a density of states at the Fermi
level concentrated at the nodes in photoemission experiments, and the antinodal
pseudogap observed by photoemission, optical conductivity, nuclear magnetic
resonance Knight shift, as well as other complementary diffraction, transport
and thermodynamic measurements. One of the possibilities considered is that the
small Fermi surface pockets observed at high magnetic fields can be understood
in terms of Fermi surface reconstruction by a form of small wavevector charge
order, observed over long lengthscales in experiments such as nuclear magnetic
resonance and x-ray scattering, potentially accompanied by an additional
mechanism to gap the antinodal density of states.
|
1112.1373v2
|
2012-01-23
|
The Star Formation History of Leo T from Hubble Space Telescope Imaging
|
We present the star formation history (SFH) of the faintest known
star-forming galaxy, Leo T, based on imaging taken with the Hubble Space
Telescope (HST) Wide Field Planetary Camera 2 (WFPC2). The HST/WFPC2
color-magnitude diagram (CMD) of Leo T is exquisitely deep, extending ~ 2
magnitudes below the oldest main sequence turnoff, permitting excellent
constraints on star formation at all ages. We use a maximum likelihood CMD
fitting technique to measure the SFH of Leo T assuming three different sets of
stellar evolution models: Padova (solar-scaled metallicity) and BaSTI (both
solar-scaled and alpha-enhanced metallicities). The resulting SFHs are
remarkably consistent at all ages, indicating that our derived SFH is robust to
the choice of stellar evolution model. From the lifetime SFH of Leo T, we find
that 50% of the total stellar mass formed prior to z ~ 1 (7.6 Gyr ago).
Subsequent to this epoch, the SFH of Leo T is roughly constant until the most
recent ~ 25 Myr, where the SFH shows an abrupt drop. This decrease could be due
to a cessation of star formation or stellar initial mass function sampling
effects, but we are unable to distinguish between the two scenarios. Overall,
our measured SFH is consistent with previously derived SFHs of Leo T. However,
the HST-based solution provides improved age resolution and reduced
uncertainties at all epochs. The SFH, baryonic gas fraction, and location of
Leo T are unlike any of the other recently discovered faint dwarf galaxies in
the Local Group, and instead bear strong resemblance to gas-rich dwarf galaxies
(irregular or transition), suggesting that gas-rich dwarf galaxies may share
common modes of star formation over a large range of stellar mass (~ 10^5-10^9
Msun).
|
1201.4859v1
|
2012-02-13
|
Internal Stellar Kinematics of M32 from the SPLASH Survey: Dark Halo Constraints and the Formation of Compact Elliptical Galaxies
|
As part of the SPLASH survey of the Andromeda galaxy (M31) and its neighbors,
we have obtained Keck/DEIMOS spectra of the compact elliptical (cE) satellite
M32. This is the first resolved-star kinematical study of any cE galaxy. In
contrast to previous studies that extended out to r<30"~1Re~100pc, we measure
the rotation curve and velocity dispersion profile out to r~250" and higher
order Gauss-Hermite moments out to r~70". We achieve this by combining
integrated-light spectroscopy at small radii (where crowding/blending are
severe) with resolved stellar spectroscopy at larger radii, using spatial and
kinematical information to statistically account for M31 contamination. The
rotation curve and velocity dispersion profile extend well beyond the radius
(r~150") where the isophotes are distorted. Unlike NGC 205, another close dwarf
companion of M31, M32's kinematic are regular and symmetric and do not show
obvious sharp gradients across the region of isophotal elongation and twists.
We interpret M32's kinematics using three-integral axisymmetric dynamical
equilibrium models constructed using Schwarzschild's orbit superposition
technique. Models with a constant M/L can fit the data remarkably well.
However, since such a model requires an increasing tangential anisotropy with
radius, invoking the presence of an extended dark halo may be more plausible.
Such an extended dark halo is definitely required to bind a half-dozen
fast-moving stars observed at the largest radii, but these stars may not be an
equilibrium component of M32. The observed regularity of the stellar
kinematics, as well as the possible detection of an extended dark halo, are
unexpected if M31 tides are significant at large radii. While these findings by
themselves do not rule out tidal models for cE formation, they suggest that
tidal stripping may not be as significant for shaping cE galaxies as has often
been argued.
|
1202.2897v3
|
2012-04-19
|
The SPLASH Survey: Kinematics of Andromeda's Inner Spheroid
|
The combination of large size, high stellar density, high metallicity, and
Sersic surface brightness profile of the spheroidal component of the Andromeda
galaxy (M31) within R_proj ~ 20 kpc suggest that it is unlike any subcomponent
of the Milky Way. In this work we capitalize on our proximity to and external
view of M31 to probe the kinematical properties of this "inner spheroid." We
employ a Markov chain Monte Carlo (MCMC) analysis of resolved stellar
kinematics from Keck/DEIMOS spectra of 5651 red giant branch stars to
disentangle M31's inner spheroid from its stellar disk. We measure the mean
velocity and dispersion of the spheroid in each of five spatial bins after
accounting for a locally cold stellar disk as well as the Giant Southern Stream
and associated tidal debris. For the first time, we detect significant spheroid
rotation (v_rot ~ 50 km/s) beyond R_proj ~ 5 kpc. The velocity dispersion
decreases from about 140 km/s at R_proj = 7 kpc to 120 km/s at R_proj = 14 kpc,
consistent to 2 sigma with existing measurements and models. We calculate the
probability that a given star is a member of the spheroid and find that the
spheroid has a significant presence throughout the spatial extent of our
sample. Lastly, we show that the flattening of the spheroid is due to velocity
anisotropy in addition to rotation. Though this suggests that the inner
spheroid of M31 more closely resembles an elliptical galaxy than a typical
spiral galaxy bulge, it should be cautioned that our measurements are much
farther out (2 - 14 r_eff) than for the comparison samples.
|
1204.4455v1
|
2012-06-07
|
KELT-1b: A Strongly Irradiated, Highly Inflated, Short Period, 27 Jupiter-mass Companion Transiting a mid-F Star
|
We present the discovery of KELT-1b, the first transiting low-mass companion
from the wide-field Kilodegree Extremely Little Telescope-North (KELT-North)
survey. The V=10.7 primary is a mildly evolved, solar-metallicity, mid-F star.
The companion is a low-mass brown dwarf or super-massive planet with mass of
27.23+/-0.50 MJ and radius of 1.110+0.037-0.024 RJ, on a very short period
(P=1.21750007) circular orbit. KELT-1b receives a large amount of stellar
insolation, with an equilibrium temperature assuming zero albedo and perfect
redistribution of 2422 K. Upper limits on the secondary eclipse depth indicate
that either the companion must have a non-zero albedo, or it must experience
some energy redistribution. Comparison with standard evolutionary models for
brown dwarfs suggests that the radius of KELT-1b is significantly inflated.
Adaptive optics imaging reveals a candidate stellar companion to KELT-1, which
is consistent with an M dwarf if bound. The projected spin-orbit alignment
angle is consistent with zero stellar obliquity, and the vsini of the primary
is consistent with tidal synchronization. Given the extreme parameters of the
KELT-1 system, we expect it to provide an important testbed for theories of the
emplacement and evolution of short-period companions, and theories of tidal
dissipation and irradiated brown dwarf atmospheres.
|
1206.1635v1
|
2012-06-12
|
A spectroscopic survey of Andromeda's Western Shelf
|
The Andromeda galaxy (M31) shows many tidal features in its halo, including
the Giant Southern Stream (GSS) and a sharp ledge in surface density on its
western side (the W Shelf). Using DEIMOS on the Keck telescope, we obtain
radial velocities of M31's giant stars along its NW minor axis, in a radial
range covering the W Shelf and extending beyond its edge. In the space of
velocity versus radius, the sample shows the wedge pattern expected from a
radial shell, which is detected clearly here for the first time. This confirms
predictions from an earlier model of formation of the GSS, which proposed that
the W Shelf is a shell from the third orbital wrap of the same tidal debris
stream that produces the GSS, with the main body of the progenitor lying in the
second wrap. We calculate the distortions in the shelf wedge pattern expected
from its outward expansion and angular momentum, and show that these effects
are echoed in the data. In addition, a hot, relatively smooth spheroid
population is clearly present. We construct a bulge-disk-halo N-body model that
agrees with surface brightness and kinematic constraints, and combine it with a
simulation of the GSS. From the contrasting kinematic signatures of the hot
spheroid and shelf components, we decompose the observed stellar metallicity
distribution into contributions from each component using a non-parametric
mixture model. The shelf component's metallicity distribution matches previous
observations of the GSS superbly, further strengthening the evidence they are
connected and bolstering the case for a massive progenitor of this stream.
|
1206.2619v1
|
2012-09-10
|
Stellar Kinematics of the Andromeda II Dwarf Spheroidal Galaxy
|
We present kinematical profiles and metallicity for the M31 dwarf spheroidal
(dSph) satellite galaxy Andromeda II (And II) based on Keck DEIMOS spectroscopy
of 531 red giant branch stars. Our kinematical sample is among the largest for
any M31 satellite and extends out to two effective radii (r_eff = 5.3' = 1.1
kpc). We find a mean systemic velocity of -192.4+-0.5 km/s and an average
velocity dispersion of sigma_v = 7.8+-1.1 km/s. While the rotation velocity
along the major axis of And II is nearly zero (<1 km/s), the rotation along the
minor axis is significant with a maximum rotational velocity of v_max=8.6+-1.8
km/s. We find a kinematical major axis, with a maximum rotational velocity of
v_max=10.9+-2.4 km/s, misaligned by 67 degrees to the isophotal major axis. And
II is thus the first dwarf galaxy with evidence for nearly prolate rotation
with a v_max/sigma_v = 1.1, although given its ellipticity of epsilon = 0.10,
this object may be triaxial. We measured metallicities for a subsample of our
data, finding a mean metallicity of [Fe/H] = -1.39+- 0.03 dex and an internal
metallicity dispersion of 0.72+-0.03 dex. We find a radial metallicity gradient
with metal-rich stars more centrally concentrated, but do not observe a
significant difference in the dynamics of two metallicity populations. And II
is the only known dwarf galaxy to show minor axis rotation making it a unique
system whose existence offers important clues on the processes responsible for
the formation of dSphs.
|
1209.2116v1
|
2012-09-11
|
The Panchromatic Hubble Andromeda Treasury II. Tracing the Inner M31 Halo with Blue Horizontal Branch Stars
|
We attempt to constrain the shape of M31's inner stellar halo by tracing the
surface density of blue horizontal branch (BHB) stars at galactocentric
distances ranging from 2 kpc to 35 kpc. Our measurements make use of resolved
stellar photometry from a section of the Panchromatic Hubble Andromeda Treasury
(PHAT) survey, supplemented by several archival Hubble Space Telescope
observations. We find that the ratio of BHB to red giant stars is relatively
constant outside of 10 kpc, suggesting that the BHB is as reliable a tracer of
the halo population as the red giant branch. In the inner halo, we do not
expect BHB stars to be produced by the high metallicity bulge and disk, making
BHB stars a good candidate to be a reliable tracer of the stellar halo to much
smaller galactocentric distances. If we assume a power-law profile r^(-\alpha)
for the 2-D projected surface density BHB distribution, we obtain a
high-quality fit with a 2-D power-law index of \alpha=2.6^{+0.3}_{-0.2} outside
of 3 kpc, which flattens to \alpha<1.2 inside of 3 kpc. This slope is
consistent with previous measurements but is anchored to a radial baseline that
extends much farther inward. Finally, assuming azimuthal symmetry and a
constant mass-to-light ratio, the best-fitting profile yields a total halo
stellar mass of 2.1^{+1.7}_{-0.4} x 10^9 M_sun. These properties are comparable
with both simulations of stellar halo formation formed by satellite disruption
alone, and with simulations that include some in situ formation of halo stars.
|
1209.2416v1
|
2012-10-23
|
Supernova Remnant Progenitor Masses in M31
|
Using HST photometry, we age-date 59 supernova remnants (SNRs) in the spiral
galaxy M31 and use these ages to estimate zero-age main sequence masses (MZAMS)
for their progenitors. To accomplish this, we create color-magnitude diagrams
(CMDs) and use CMD fitting to measure the recent star formation history (SFH)
of the regions surrounding cataloged SNR sites. We identify any young coeval
population that likely produced the progenitor star and assign an age and
uncertainty to that population. Application of stellar evolution models allows
us to infer the MZAMS from this age. Because our technique is not contingent on
precise location of the progenitor star, it can be applied to the location of
any known SNR. We identify significant young SF around 53 of the 59 SNRs and
assign progenitor masses to these, representing a factor of 2 increase over
currently measured progenitor masses. We consider the remaining 6 SNRs as
either probable Type Ia candidates or the result of core-collapse progenitors
that have escaped their birth sites. The distribution of recovered progenitor
masses is bottom heavy, showing a paucity of the most massive stars. If we
assume a single power law distribution, dN/dM proportional to M^alpha, we find
a distribution that is steeper than a Salpeter IMF (alpha=-2.35). In
particular, we find values of alpha outside the range -2.7 to -4.4 inconsistent
with our measured distribution at 95% confidence. If instead we assume a
distribution that follows a Salpeter IMF up to some maximum mass, we find that
values of M_max greater than 26 Msun are inconsistent with the measured
distribution at 95% confidence. In either scenario, the data suggest that some
fraction of massive stars may not explode. The result is preliminary and
requires more SNRs and further analysis. In addition, we use our distribution
to estimate a minimum mass for core collapse between 7.0 and 7.8 Msun.
|
1210.6353v1
|
2012-11-02
|
Sketched SVD: Recovering Spectral Features from Compressive Measurements
|
We consider a streaming data model in which n sensors observe individual
streams of data, presented in a turnstile model. Our goal is to analyze the
singular value decomposition (SVD) of the matrix of data defined implicitly by
the stream of updates. Each column i of the data matrix is given by the stream
of updates seen at sensor i. Our approach is to sketch each column of the
matrix, forming a "sketch matrix" Y, and then to compute the SVD of the sketch
matrix. We show that the singular values and right singular vectors of Y are
close to those of X, with small relative error. We also believe that this bound
is of independent interest in non-streaming and non-distributed data collection
settings.
Assuming that the data matrix X is of size Nxn, then with m linear
measurements of each column of X, we obtain a smaller matrix Y with dimensions
mxn. If m = O(k \epsilon^{-2} (log(1/\epsilon) + log(1/\delta)), where k
denotes the rank of X, then with probability at least 1-\delta, the singular
values \sigma'_j of Y satisfy the following relative error result
(1-\epsilon)^(1/2)<= \sigma'_j/\sigma_j <= (1 + \epsilon)^(1/2) as compared
to the singular values \sigma_j of the original matrix X. Furthermore, the
right singular vectors v'_j of Y satisfy
||v_j-v_j'||_2 <= min(sqrt{2},
(\epsilon\sqrt{1+\epsilon})/(\sqrt{1-\epsilon}) max_{i\neq j}
(\sqrt{2}\sigma_i\sigma_j)/(min_{c\in[-1,1]}(|\sigma^2_i-\sigma^2_j(1+c\epsilon)|)))
as compared to the right singular vectors v_j of X. We apply this result to
obtain a streaming graph algorithm to approximate the eigenvalues and
eigenvectors of the graph Laplacian in the case where the graph has low rank
(many connected components).
|
1211.0361v1
|
2012-11-05
|
KELT-3b: A Hot Jupiter Transiting a V=9.8 Late-F Star
|
We report the discovery of KELT-3b, a moderately inflated transiting hot
Jupiter with a mass of 1.477 (-0.067, +0.066) M_J, and radius of 1.345 +/-
0.072 R_J, with an orbital period of 2.7033904 +/- 0.000010 days. The host
star, KELT-3, is a V=9.8 late F star with M_* = 1.278 (-0.061, +0.063) M_sun,
R_* = 1.472 (-0.067, +0.065) R_sun, T_eff = 6306 (-49, +50) K, log(g) = 4.209
(-0.031, +0.033), and [Fe/H] = 0.044 (-0.082, +0.080), and has a likely proper
motion companion. KELT-3b is the third transiting exoplanet discovered by the
KELT survey, and is orbiting one of the 20 brightest known transiting planet
host stars, making it a promising candidate for detailed characterization
studies. Although we infer that KELT-3 is significantly evolved, a preliminary
analysis of the stellar and orbital evolution of the system suggests that the
planet has likely always received a level of incident flux above the
empirically-identified threshold for radius inflation suggested by Demory &
Seager (2011).
|
1211.1031v2
|
2013-01-16
|
Mandated data archiving greatly improves access to research data
|
The data underlying scientific papers should be accessible to researchers
both now and in the future, but how best can we ensure that these data are
available? Here we examine the effectiveness of four approaches to data
archiving: no stated archiving policy, recommending (but not requiring)
archiving, and two versions of mandating data deposition at acceptance. We
control for differences between data types by trying to obtain data from papers
that use a single, widespread population genetic analysis, STRUCTURE. At one
extreme, we found that mandated data archiving policies that require the
inclusion of a data availability statement in the manuscript improve the odds
of finding the data online almost a thousand-fold compared to having no policy.
However, archiving rates at journals with less stringent policies were only
very slightly higher than those with no policy at all. At one extreme, we found
that mandated data archiving policies that require the inclusion of a data
availability statement in the manuscript improve the odds of finding the data
online almost a thousand fold compared to having no policy. However, archiving
rates at journals with less stringent policies were only very slightly higher
than those with no policy at all. We also assessed the effectiveness of asking
for data directly from authors and obtained over half of the requested
datasets, albeit with about 8 days delay and some disagreement with authors.
Given the long term benefits of data accessibility to the academic community,
we believe that journal based mandatory data archiving policies and mandatory
data availability statements should be more widely adopted.
|
1301.3744v1
|
2013-01-25
|
Multi-site campaign for transit timing variations of WASP-12 b: possible detection of a long-period signal of planetary origin
|
The transiting planet WASP-12 b was identified as a potential target for
transit timing studies because a departure from a linear ephemeris was reported
in the literature. Such deviations could be caused by an additional planet in
the system. We attempt to confirm the existence of claimed variations in
transit timing and interpret its origin. We organised a multi-site campaign to
observe transits by WASP-12 b in three observing seasons, using 0.5-2.6-metre
telescopes. We obtained 61 transit light curves, many of them with
sub-millimagnitude precision. The simultaneous analysis of the best-quality
datasets allowed us to obtain refined system parameters, which agree with
values reported in previous studies. The residuals versus a linear ephemeris
reveal a possible periodic signal that may be approximated by a sinusoid with
an amplitude of 0.00068+/-0.00013 d and period of 500+/-20 orbital periods of
WASP-12 b. The joint analysis of timing data and published radial velocity
measurements results in a two-planet model which better explains observations
than single-planet scenarios. We hypothesize that WASP-12 b might be not the
only planet in the system and there might be the additional 0.1 M_Jup body on a
3.6-d eccentric orbit. A dynamical analysis indicates that the proposed
two-planet system is stable over long timescales.
|
1301.5976v1
|
2013-03-20
|
Genomic Sequence Diversity and Population Structure of Saccharomyces cerevisiae Assessed by RAD-seq
|
The budding yeast Saccharomyces cerevisiae is important for human food
production and as a model organism for biological research. The genetic
diversity contained in the global population of yeast strains represents a
valuable resource for a number of fields, including genetics, bioengineering,
and studies of evolution and population structure. Here, we apply a
multiplexed, reduced genome sequencing strategy (known as RAD-seq) to genotype
a large collection of S. cerevisiae strains, isolated from a wide range of
geographical locations and environmental niches. The method permits the
sequencing of the same 1% of all genomes, producing a multiple sequence
alignment of 116,880 bases across 262 strains. We find diversity among these
strains is principally organized by geography, with European, North American,
Asian and African/S. E. Asian populations defining the major axes of genetic
variation. At a finer scale, small groups of strains from cacao, olives and
sake are defined by unique variants not present in other strains. One
population, containing strains from a variety of fermentations, exhibits high
levels of heterozygosity and mixtures of alleles from European and Asian
populations, indicating an admixed origin for this group. In the context of
this global diversity, we demonstrate that a collection of seven strains
commonly used in the laboratory encompasses only one quarter of the genetic
diversity present in the full collection of strains, underscoring the
relatively limited genetic diversity captured by the current set of lab
strains. We propose a model of geographic differentiation followed by
human-associated admixture, primarily between European and Asian populations
and more recently between European and North American populations. The large
collection of genotyped yeast strains characterized here will provide a useful
resource for the broad community of yeast researchers.
|
1303.4835v1
|
2013-04-02
|
Modeling Multi-Magnet Networks Interacting Via Spin Currents
|
The significant experimental advances of the last few decades in dealing with
the interaction of spin currents and nanomagnets, at the device level, has
allowed envisioning a broad class of devices that propose to implement
information processing using spin currents and nanomagnets. To analyze such
spin-magnet logic circuits, in general, we have developed a coupled
spin-transport/ magnetization-dynamics simulation framework that could be
broadly applicable to various classes of spin-valve/ spin-torque devices.
Indeed, the primary purpose of this chapter is to describe in detail, the
overall approach we have developed to include a description of spin transport
coupled with magnetization dynamics and to show how it was benchmarked against
available data on experiments.
We address non-collinear spin-transport in Section-2 using a lumped
"4-component spin-circuit formalism" that describes the interaction of
non-collinear magnets (required for modeling spin torque), by computing
4-component currents and voltages at every node of a "circuit". For modeling
the magnetization dynamics, we use the standard Landau-Lifshitz-Gilbert (LLG)
equation with the Slonczewski and the field-like terms included for spin
torque. Section-3 describes how this LLG model is coupled with the spin
transport model to analyze spin-torque experiments and spin-magnet circuits in
general.
We include MATLAB codes in the Appendix to facilitate a "hands-on"
understanding of our model and hope it will enable interested readers to
conveniently analyze their own experiments, develop a deeper insight into
spin-magnet circuits or come up with their own creative designs.
|
1304.0742v3
|
2013-04-03
|
A Novel Frank-Wolfe Algorithm. Analysis and Applications to Large-Scale SVM Training
|
Recently, there has been a renewed interest in the machine learning community
for variants of a sparse greedy approximation procedure for concave
optimization known as {the Frank-Wolfe (FW) method}. In particular, this
procedure has been successfully applied to train large-scale instances of
non-linear Support Vector Machines (SVMs). Specializing FW to SVM training has
allowed to obtain efficient algorithms but also important theoretical results,
including convergence analysis of training algorithms and new characterizations
of model sparsity.
In this paper, we present and analyze a novel variant of the FW method based
on a new way to perform away steps, a classic strategy used to accelerate the
convergence of the basic FW procedure. Our formulation and analysis is focused
on a general concave maximization problem on the simplex. However, the
specialization of our algorithm to quadratic forms is strongly related to some
classic methods in computational geometry, namely the Gilbert and MDM
algorithms.
On the theoretical side, we demonstrate that the method matches the
guarantees in terms of convergence rate and number of iterations obtained by
using classic away steps. In particular, the method enjoys a linear rate of
convergence, a result that has been recently proved for MDM on quadratic forms.
On the practical side, we provide experiments on several classification
datasets, and evaluate the results using statistical tests. Experiments show
that our method is faster than the FW method with classic away steps, and works
well even in the cases in which classic away steps slow down the algorithm.
Furthermore, these improvements are obtained without sacrificing the predictive
accuracy of the obtained SVM model.
|
1304.1014v2
|
2013-07-10
|
Hard X-ray and ultraviolet emission during the 2011 June 7 solar flare
|
The relationship between X-ray and UV emission during flares, particularly in
the context of quasi-periodic pulsations, remains unclear. To address this, we
study the impulsive X-ray and UV emission during the eruptive flare of 2011
June 7 utilising X-ray imaging from RHESSI and UV 1700A imaging from SDO/AIA.
This event is associated with quasi-periodic pulsations in X-ray and possibly
UV emission, as well as substantial parallel and perpendicular motion of the
hard X-ray footpoints.
The motion of the footpoints parallel to the flare ribbons is unusual; it is
shown to reverse direction on at least two occasions. However, there is no
associated short-timescale motion of the UV bright regions. Additionally, we
find that the locations of the brightest X-ray and UV regions are different,
particularly during the early portion of the flare impulsive phase, despite
their integrated emission being strongly correlated in time. Correlation
analysis of measured flare properties, such as the footpoint separation, flare
shear, photospheric magnetic field and coronal reconnection rate, reveals that
- in the impulsive phase - the 25 - 50 keV hard X-ray flux is only weakly
correlated with these properties, in contrast to previous studies.
We characterise this event in terms of long-term behaviour, where the X-ray
nonthermal, thermal, and UV emission sources appear temporally and spatially
consistent, and short-term behaviour, where the emission sources are
inconsistent and quasi-periodic pulsations are a dominant feature requiring
explanation. We suggest that the short timescale behaviour of hard X-ray
footpoints, and the nature of the observed quasi-periodic pulsations, is
determined by fundamental, as-yet unobserved properties of the reconnection
region and particle acceleration sites. This presents a challenge for current
three-dimensional flare reconnection models.
|
1307.2874v2
|
2013-07-11
|
Inferring the Andromeda Galaxy's mass from its giant southern stream with Bayesian simulation sampling
|
M31 has a giant stream of stars extending far to the south and a great deal
of other tidal debris in its halo, much of which is thought to be directly
associated with the southern stream. We model this structure by means of
Bayesian sampling of parameter space, where each sample uses an N-body
simulation of a satellite disrupting in M31's potential. We combine constraints
on stellar surface densities from the Isaac Newton Telescope survey of M31 with
kinematic data and photometric distances. This combination of data tightly
constrains the model, indicating a stellar mass at last pericentric passage of
log(M_s / Msun) = 9.5+-0.1, comparable to the LMC. Any existing remnant of the
satellite is expected to lie in the NE Shelf region beside M31's disk, at
velocities more negative than M31's disk in this region. This rules out the
prominent satellites M32 or NGC 205 as the progenitor, but an overdensity
recently discovered in M31's NE disk sits at the edge of the progenitor
locations found in the model. M31's virial mass is constrained in this model to
be log(M200) = 12.3+-0.1, alleviating the previous tension between
observational virial mass estimates and expectations from the general galactic
population and the timing argument. The techniques used in this paper, which
should be more generally applicable, are a powerful method of extracting
physical inferences from observational data on tidal debris structures.
|
1307.3219v1
|
2013-07-31
|
Learning about SANS Instruments and Data Reduction from Round Robin Measurements on Samples of Polystyrene Latex
|
Measurements of a well-characterised standard sample can verify the
performance of an instrument. Typically, small-angle neutron scattering
instruments are used to investigate a wide range of samples and may often be
used in a number of configurations. Appropriate standard samples are useful to
test different aspects of the performance of hardware as well as that of the
data reduction and analysis software. Measurements on a number of instruments
with different intrinsic characteristics and designs in a round robin can not
only better characterise the performance for a wider range of conditions but
also, perhaps more importantly, reveal the limits of the current state of the
art of small-angle scattering. The exercise, followed by detailed analysis,
tests the limits of current understanding as well as uncovers often forgotten
assumptions, simplifications and approximations that underpin the current
practice of the technique. This paper describes measurements of polystyrene
latex, radius 72 nm with a number of instruments. Scattering from monodisperse,
uniform spherical particles is simple to calculate and displays sharp minima.
Such data test the calibrations of intensity, wavelength and resolution as well
as the detector response. Smoothing due to resolution, multiple scattering and
polydispersity has been determined. Sources of uncertainty are often related to
systematic deviations and calibrations rather than random counting errors. The
study has prompted development of software to treat modest multiple scattering
and to better model the instrument resolution. These measurements also allow
checks of data reduction algorithms and have identified how they can be
improved. The reproducibility and the reliability of instruments and the
accuracy of parameters derived from the data are described.
|
1307.8296v1
|
2013-08-09
|
A Search for RR Lyrae Stars in Segue 2 and Segue 3
|
We present an extensive search for RR Lyrae stars in and around the
ultra-faint Milky Way companions Segue 2 and Segue 3. The former (M_V = -2.5,
Belokurov et al. 2009) appears to be an extremely faint dwarf galaxy companion
of the Milky Way. The latter (M_V = 0.0, Fadely et al. 2011) is among the
faintest star clusters known. We use B and V band time-series imaging obtained
at the WIYN 0.9 meter telescope at Kitt Peak National Observatory to search for
RR Lyrae in these objects. In our Segue 2 observations, we present a previously
unknown fundamental mode (RRab) RR Lyrae star with a period of P_ab = 0.748
days. With this measurement, we revisit the inverse correlation between <P_ab>
and <[Fe/H]> established in the literature for Milky Way dwarf galaxies and
their RR Lyrae. In this context, the long period of Segue 2's RRab star as well
as the known significant spread in metallicity in this dwarf galaxy are
consistent with the observed trend in <P_ab> and <[Fe/H]>. We derive the first
robust distance to Segue 2, using both its RRab star and spectroscopically
confirmed blue horizontal branch stars. Using [Fe/H] = -2.16 and -2.44 dex, we
find d_RRL = 36.6 +2.5/-2.4 and 37.7 +/- 2.7 kpc; assuming [Fe/H] = -2.257 dex,
we find d_BHB = 34.4 +/- 2.6 kpc. Although no RR Lyrae were present in the
Segue 3 field, we found a candidate eclipsing binary star system.
|
1308.2227v1
|
2013-10-15
|
A New Approach to Detailed Structural Decomposition from the SPLASH and PHAT Surveys: Kicked-up Disk Stars in the Andromeda Galaxy?
|
We characterize the bulge, disk, and halo subcomponents in the Andromeda
galaxy (M31) over the radial range 4 < R_proj < 225 kpc. The cospatial nature
of these subcomponents renders them difficult to disentangle using surface
brightness (SB) information alone, especially interior to ~20 kpc. Our new
decomposition technique combines information from the luminosity function (LF)
of over 1.5 million bright (20 < m_814W < 22) stars from the Panchromatic
Hubble Andromeda Treasury (PHAT) survey, radial velocities of over 5000 red
giant branch stars in the same magnitude range from the Spectroscopic and
Photometric Landscape of Andromeda's Stellar Halo (SPLASH) survey, and
integrated I-band SB profiles from various sources. We use an affine-invariant
Markov chain Monte Carlo algorithm to fit an appropriate toy model to these
three data sets. The bulge, disk, and halo SB profiles are modeled as a Sersic,
exponential, and cored power-law, respectively, and the LFs are modeled as
broken power-laws. We present probability distributions for each of 32
parameters describing the SB profiles and LFs of the three subcomponents. We
find that the number of stars with a disk-like LF is ~5% larger than the the
number with disk-like (dynamically cold) kinematics, suggesting that some stars
born in the disk have been dynamically heated to the point that they are
kinematically indistinguishable from halo members. This is the first
kinematical evidence for a "kicked-up disk" halo population in M31. The
fraction of kicked-up disk stars is consistent with that found in simulations.
We also find evidence for a radially varying disk LF, consistent with a
negative metallicity gradient in the stellar disk.
|
1310.4179v1
|
2013-12-11
|
The Nosoi commute: a spatial perspective on the rise of BSL-4 laboratories in cities
|
Recent H5N1 influenza research has revived the debate on the storage and
manipulation of potentially harmful pathogens. In the last two decades, new
high biosafety (BSL-4) laboratories entered into operation, raising strong
concerns from the public. The probability of an accidental release of a
pathogen from a BSL-4 laboratory is extremely low, but the corresponding risk
-- defined as the probability of occurrence multiplied by its impact -- could
be significant depending on the pathogen specificities and the population
potentially affected. A list of BSL-4 laboratories throughout the world, with
their location and date of first activity, was established from publicly
available sources. This database was used to estimate the total population
living within a daily commuting distance of BSL-4 laboratories, and to quantify
how this figure changed over time. We show that from 1990 to present, the
population living within the commuting belt of BSL-4 laboratories increased by
a factor of 4 to reach up to 1.8% of the world population, owing to an increase
in the number of facilities and their installation in cities. Europe is
currently hosting the largest population living in the direct vicinity of BSL-4
laboratories, while the recent building of new facilities in Asia suggests that
an important increase of the population living close to BSL-4 laboratories will
be observed in the next decades. We discuss the potential implications in term
of global risk, and call for better pathogen-specific quantitative assessment
of the risk of outbreaks resulting from the accidental release of potentially
pandemic pathogens
|
1312.3283v2
|
2013-12-18
|
The thermal Sunyaev Zel'dovich effect power spectrum in light of Planck
|
(Abridged) The amplitude of the thermal Sunyaev Zel'dovich effect (tSZ) power
spectrum is extremely sensitive to the abundance of galaxy clusters and
therefore to fundamental cosmological parameters that control their growth,
such as sigma_8 and Omega_m. Here we explore the sensitivity of the tSZ power
spectrum to important non-gravitational ('sub-grid') physics by employing the
cosmo-OWLS suite of large-volume cosmological hydrodynamical simulations, run
in both the Planck and WMAP7 best-fit cosmologies. On intermediate and small
angular scales (ell > ~1000, or theta < ~10 arcmin), accessible with the South
Pole Telescope and the Atacama Cosmology Telescope, the predicted tSZ power
spectrum is highly model dependent, with AGN feedback having a particularly
large effect. However, at large scales, observable with the Planck telescope,
the effects of sub-grid physics are minor. Comparing the simulations with
observations, we find a significant amplitude offset on all measured angular
scales (including large scales), if the Planck best-fit cosmology is assumed by
the simulations. This is shown to be a generic result for all current tSZ
models. By contrast, if the WMAP7 cosmology is adopted, there is full
consistency with the Planck power spectrum measurements on large scales and
agreement at the 2 sigma level with the SPT/ACT measurements at intermediate
scales for our fiducial AGN model, which Le Brun et al. (2014) have shown
reproduces the 'resolved' properties of the local cluster population remarkably
well. These findings strongly suggest that there are significantly fewer
massive galaxy clusters than expected for the Planck best-fit cosmology, which
is consistent with recent measurements of the tSZ number counts. Our findings
therefore pose a significant challenge to the cosmological parameter values
preferred (and/or the model adopted) by the Planck primary CMB analyses.
|
1312.5341v2
|
2014-02-21
|
How to Scale Exponential Backoff
|
Randomized exponential backoff is a widely deployed technique for
coordinating access to a shared resource. A good backoff protocol should,
arguably, satisfy three natural properties: (i) it should provide constant
throughput, wasting as little time as possible; (ii) it should require few
failed access attempts, minimizing the amount of wasted effort; and (iii) it
should be robust, continuing to work efficiently even if some of the access
attempts fail for spurious reasons. Unfortunately, exponential backoff has some
well-known limitations in two of these areas: it provides poor (sub-constant)
throughput (in the worst case), and is not robust (to resource acquisition
failures).
The goal of this paper is to "fix" exponential backoff by making it scalable,
particularly focusing on the case where processes arrive in an on-line,
worst-case fashion. We present a relatively simple backoff
protocol~Re-Backoff~that has, at its heart, a version of exponential backoff.
It guarantees expected constant throughput with dynamic process arrivals and
requires only an expected polylogarithmic number of access attempts per
process.
Re-Backoff is also robust to periods where the shared resource is unavailable
for a period of time. If it is unavailable for $D$ time slots, Re-Backoff
provides the following guarantees. When the number of packets is a finite $n$,
the average expected number of access attempts for successfully sending a
packet is $O(\log^2( n + D))$. In the infinite case, the average expected
number of access attempts for successfully sending a packet is $O( \log^2(\eta)
+ \log^2(D) )$ where $\eta$ is the maximum number of processes that are ever in
the system concurrently.
|
1402.5207v4
|
2014-04-10
|
The Spitzer South Pole Telescope Deep Field Survey: Linking galaxies and halos at z=1.5
|
We present an analysis of the clustering of high-redshift galaxies in the
recently completed 94 deg$^2$ Spitzer-SPT Deep Field survey. Applying flux and
color cuts to the mid-infrared photometry efficiently selects galaxies at
$z\sim1.5$ in the stellar mass range $10^{10}-10^{11}M_\odot$, making this
sample the largest used so far to study such a distant population. We measure
the angular correlation function in different flux-limited samples at scales
$>6^{\prime \prime}$ (corresponding to physical distances $>0.05$ Mpc) and
thereby map the one- and two-halo contributions to the clustering. We fit halo
occupation distributions and determine how the central galaxy's stellar mass
and satellite occupation depend on the halo mass. We measure a prominent peak
in the stellar-to-halo mass ratio at a halo mass of $\log(M_{\rm halo} /
M_\odot) = 12.44\pm0.08$, 4.5 times higher than the $z=0$ value. This supports
the idea of an evolving mass threshold above which star formation is quenched.
We estimate the large-scale bias in the range $b_g=2-4$ and the satellite
fraction to be $f_\mathrm{sat}\sim0.2$, showing a clear evolution compared to
$z=0$. We also find that, above a given stellar mass limit, the fraction of
galaxies that are in similar mass pairs is higher at $z=1.5$ than at $z=0$. In
addition, we measure that this fraction mildly increases with the stellar mass
limit at $z=1.5$, which is the opposite of the behavior seen at low-redshift.
|
1404.2930v2
|
2014-04-28
|
The Star Formation Histories of Local Group Dwarf Galaxies I. Hubble Space Telescope / Wide Field Planetary Camera 2 Observations
|
We present uniformly measured star formation histories (SFHs) of 40 Local
Group dwarf galaxies based on color-magnitude diagram (CMD) analysis from
archival Hubble Space Telescope imaging. We demonstrate that accurate SFHs can
be recovered from CMDs that do not reach the oldest main sequence turn-off
(MSTO), but emphasize that the oldest MSTO is critical for precisely
constraining the earliest epochs of star formation. We find that: (1) the
average lifetime SFHs of dwarf spheroidals (dSphs) can be approximated by an
exponentially declining SFH with $\tau$ $\sim$ 5 Gyr; (2) lower luminosity
dSphs are less likely to have extended SFHs than more luminous dSphs; (3) the
average SFHs of dwarf irregulars (dIrrs), transition dwarfs (dTrans), and dwarf
ellipticals (dEs) can be approximated by the combination of an exponentially
declining SFH ($\tau$ $\sim$ 3-4 Gyr) for lookback ages $>$ 10-12 Gyr ago and a
constant SFH thereafter; (4) the observed fraction of stellar mass formed prior
to z=2 ranges considerably (80\% for galaxies with M $<$ 10$^5$ M$_{\odot}$ to
30\% for galaxies with M$>$10$^7$ M$_{\odot}$) and is largely explained by
environment; (5) the distinction between "ultra-faint" and "classical" dSphs is
arbitrary; (6) LG dIrrs formed a significantly higher fraction of stellar mass
prior to z=2 than the SDSS galaxies from Leiter 2012 and the SFHs from the
abundance matching models of Behroozi et al. 2013. This may indicate higher
than expected star-formation efficiencies at early times in low mass galaxies.
Finally, we provide all the SFHs in tabulated electronic format for use by the
community.
|
1404.7144v1
|
2014-06-26
|
Spin Supercurrent, Magnetization Dynamics, and Phi-State in Spin-Textured Josephson Junctions
|
The prospect of combining the dissipationless nature of superconducting
currents with the spin-polarization of magnetic materials is interesting with
respect to exploring superconducting analogues of topics in spintronics. In
order to accomplish this aim, it is pivotal to understand how
spin-supercurrents interact dynamically with magnetization textures. We
investigate the appearance of a spin-supercurrent and the resulting
magnetization dynamics in a textured magnetic Josephson current by using three
experimentally relevant models: i) a S/F/S junction with spin-active
interfaces, ii) a S/F1/F2/F3/S Josephson junction with a ferromagnetic
trilayer, and iii) a Josephson junction containing a domain wall. In all of
these cases, the supercurrent is spin-polarized and exerts a spin-transfer
torque on the ferromagnetic interlayers which causes magnetization dynamics.
Using a scattering matrix formalism in the clean limit, we compute the
Andreev-bound states and free energy of the system which is used to solve the
Landau-Lifshiftz-Gilbert equation. We compute both how the inhomogeneous
magnetism influences the phase-dependence of the charge supercurrent as well as
the magnetization dynamics caused by the spin-supercurrent. Using a realistic
experimental parameter set, we find that the supercurrent can induce
magnetization switching that is controlled by the superconducting phase
difference. Moreover, we demonstrate that the combined effect of chiral spin
symmetry breaking and interface scattering causes the system to act as a phase
battery that may supply any superconducting phase difference phi in the ground
state. Such a phi junction is accompanied by an anomalous supercurrent
appearing even at zero phase difference, and we demonstrate that the flow
direction of this current is controlled by the chirality of the magnetization
configuration.
|
1406.7016v1
|
2014-07-11
|
Digital frequency domain multiplexing readout electronics for the next generation of millimeter telescopes
|
Frequency domain multiplexing (fMux) is an established technique for the
readout of transition-edge sensor (TES) bolometers in millimeter-wavelength
astrophysical instrumentation. In fMux, the signals from multiple detectors are
read out on a single pair of wires reducing the total cryogenic thermal loading
as well as the cold component complexity and cost of a system. The current
digital fMux system, in use by POLARBEAR, EBEX, and the South Pole Telescope,
is limited to a multiplexing factor of 16 by the dynamic range of the
Superconducting Quantum Interference Device pre-amplifier and the total system
bandwidth. Increased multiplexing is key for the next generation of large
format TES cameras, such as SPT-3G and POLARBEAR2, which plan to have on the of
order 15,000 detectors.
Here, we present the next generation fMux readout, focusing on the warm
electronics. In this system, the multiplexing factor increases to 64 channels
per module (2 wires) while maintaining low noise levels and detector stability.
This is achieved by increasing the system bandwidth, reducing the dynamic range
requirements though active feedback, and digital synthesis of voltage biases
with a novel polyphase filter algorithm. In addition, a version of the new fMux
readout includes features such as low power consumption and radiation-hard
components making it viable for future space-based millimeter telescopes such
as the LiteBIRD satellite.
|
1407.3161v1
|
2014-08-03
|
Bounded Real Lemma and structured singular value versus diagonal scaling: the free noncommutative setting
|
The structured singular value $\mu$ was introduced independently by Doyle and
Safanov as a tool for analyzing robustness of system stability and performance
in the presence of structured uncertainty in the system parameters. While the
structured singular value provides a necessary and sufficient criterion for
robustness with respect to a structured ball of uncertainty, it is notoriously
difficult to actually compute. The method of diagonal (or simply "D") scaling,
on the other hand, provides an easily computable upper bound (which we call
$\hat \mu$) for the structured singular value, but provides an exact evaluation
of $\mu$ (or even a useful upper bound for $\mu$) only in special cases.
However it was discovered in the 1990s that a certain enhancement of the
uncertainty structure (i.e., letting the uncertainty parameters be freely
noncommuting linear operators on an infinite-dimensional separable Hilbert
space) resulted in the $D$-scaling procedure leading to an exact evaluation of
$\mu_{\text{enhanced}}$ ($\mu_{\text{enhanced}} = \hat \mu$), at least for the
tractable special cases which were analyzed in complete detail. On the one hand
this enhanced uncertainty has some appeal from the physical point of view: one
can allow the uncertainty in the plant parameters to be time-varying, or more
generally, one can catch the uncertainty caused by the designer's decision not
to model the more complex (e.g. nonlinear) dynamics of the true plant. On the
other hand, the precise mathematical formulation of this enhanced uncertainty
structure makes contact with developments in the growing theory of analytic
functions in freely noncommuting arguments and associated formal power series
in freely noncommuting indeterminates. In this article we obtain the
$\widetilde \mu = \hat \mu$ theorem for a more satisfactory general setting.
|
1408.0519v1
|
2014-09-02
|
The Panchromatic Hubble Andromeda Treasury X. Ultraviolet to Infrared Photometry of 117 Million Equidistant Stars
|
We have measured stellar photometry with the Hubble Space Telescope (HST)
Wide Field Camera 3 (WFC3) and Advanced Camera for Surveys (ACS) in near
ultraviolet (F275W, F336W), optical (F475W, F814W), and near infrared (F110W,
F160W) bands for 117 million resolved stars in M31. As part of the Panchromatic
Hubble Andromeda Treasury (PHAT) survey, we measured photometry with
simultaneous point spread function fitting across all bands and at all source
positions after precise astrometric image alignment (<5-10 milliarcsecond
accuracy). In the outer disk, the photometry reaches a completeness-limited
depth of F475W~28, while in the crowded, high surface brightness bulge, the
photometry reaches F475W~25. We find that simultaneous photometry and optimized
measurement parameters significantly increase the detection limit of the lowest
resolution filters (WFC3/IR) providing color-magnitude diagrams that are up to
2.5 magnitudes deeper when compared with color-magnitude diagrams from WFC3/IR
photometry alone. We present extensive analysis of the data quality including
comparisons of luminosity functions and repeat measurements, and we use
artificial star tests to quantify photometric completeness, uncertainties and
biases. We find that largest sources of systematic error in the photometry are
due to spatial variations in the point spread function models and charge
transfer efficiency corrections. This stellar catalog is the largest ever
produced for equidistant sources, and is publicly available for download by the
community.
|
1409.0899v1
|
2014-09-30
|
The Supernova Progenitor Mass Distributions of M31 and M33: Further Evidence for an Upper Mass Limit
|
Using Hubble Space Telescope (HST) photometry to measure star formation
histories, we age-date the stellar populations surrounding supernova remnants
(SNRs) in M31 and M33. We then apply stellar evolution models to the ages to
infer the corresponding masses for their supernova progenitor stars. We analyze
33 M33 SNR progenitors and 29 M31 SNR progenitors in this work. We then combine
these measurements with 53 previously published M31 SNR progenitor measurements
to bring our total number of progenitor mass estimates to 115. To quantify the
mass distributions, we fit power laws of the form $dN/dM \propto M^{-\alpha}$.
Our new, larger sample of M31 progenitors follows a distribution with $\alpha =
4.4\pm 0.4$, and the M33 sample follows a distribution with $\alpha =
3.8^{+0.4}_{-0.5}$. Thus both samples are consistent within the uncertainties,
and the full sample across both galaxies gives $\alpha = 4.2\pm 0.3$. Both the
individual and full distributions display a paucity of massive stars when
compared to a Salpeter initial mass function (IMF), which we would expect to
observe if all massive stars exploded as SN that leave behind observable SNR.
If we instead fix $\alpha = 2.35$ and treat the maximum mass as a free
parameter, we find $M_{max} \sim 35-45M_{sun}$, indicative of a potential
maximum cutoff mass for SN production. Our results suggest that either SNR
surveys are biased against finding objects in the youngest (<10 Myr old)
regions, or the highest mass stars do not produce SNe.
|
1410.0018v1
|
2014-10-17
|
Ferromagnetic resonance in $ε$-Co magnetic composites
|
We investigate the electromagnetic properties of assemblies of nanoscale
$\epsilon$-cobalt crystals with size range between 5 nm to 35 nm, embedded in a
polystyrene (PS) matrix, at microwave (1-12 GHz) frequencies. We investigate
the samples by transmission electron microscopy (TEM) imaging, demonstrating
that the particles aggregate and form chains and clusters. By using a broadband
coaxial-line method, we extract the magnetic permeability in the frequency
range from 1 to 12 GHz, and we study the shift of the ferromagnetic resonance
with respect to an externally applied magnetic field. We find that the
zero-magnetic field ferromagnetic resonant peak shifts towards higher
frequencies at finite magnetic fields, and the magnitude of complex
permeability is reduced. At fields larger than 2.5 kOe the resonant frequency
changes linearly with the applied magnetic field, demonstrating the transition
to a state in which the nanoparticles become dynamically decoupled. In this
regime, the particles inside clusters can be treated as non-interacting, and
the peak position can be predicted from Kittel's ferromagnetic resonance theory
for non-interacting uniaxial spherical particles combined with the
Landau-Lifshitz-Gilbert (LLG) equation. In contrast, at low magnetic fields
this magnetic order breaks down and the resonant frequency in zero magnetic
field reaches a saturation value reflecting the interparticle interactions as
resulting from aggregation. Our results show that the electromagnetic
properties of these composite materials can be tuned by external magnetic
fields and by changes in the aggregation structure.
|
1410.4789v2
|
2014-12-28
|
The structure of solar radio noise storms
|
The Nan\c{c}ay Radioheliograph (NRH) routinely produces snapshot images of
the full sun at frequencies between 150 and 450 MHz, with typical resolution 3
arcmin and time cadence 0.2 s. Combining visibilities from the NRH and from the
Giant Meterwave Radio Telescope (GMRT) allows us to produce images of the sun
at 236 or 327 MHz, with a large FOV, high resolution and time cadence. We seek
to investigate the structure of noise storms (the most common non-thermal solar
radio emission). We focus on the relation of position and altitude of noise
storms with the observing frequency and on the lower limit of their sizes. We
present results for noise storms on four days. The results consist of an
extended halo and of one or several compact cores with relative intensity
changing over a few seconds. We found that core sizes can be almost stable over
one hour, with a minimum in the range 31-35 arcsec (less than previously
reported) and can be stable over one hour. The heliocentric distances of noise
storms are $\sim 1.20$ and 1.35 $R_{\odot}$ at 432 and 150 MHz, respectively.
Regions where storms originate are thus much denser than the ambient corona and
their vertical extent is found to be less than expected from hydrostatic
equilibrium. The smallest observed sizes impose upper limits on broadening
effects due to scattering on density inhomogeneities in the low and medium
corona and constrain the level of density turbulence in the solar corona. It is
possible that scatter broadening has been overestimated in the past, and that
the observed sizes cannot only be attributed to scattering. The vertical
structure of the noise storms is difficult to reconcile with the classical
columnar model.
|
1412.8189v2
|
2014-12-28
|
Hopf Algebras and Markov Chains
|
This thesis introduces a way to build Markov chains out of Hopf algebras. The
transition matrix of a "Hopf-power Markov chain" is (the transpose of) the
matrix of the coproduct-then-product operator on a combinatorial Hopf algebra
with respect to a suitable basis. These chains describe the
breaking-then-recombining of the combinatorial objects in the Hopf algebra. The
motivating example is the famous Gilbert-Shannon-Reeds model of
riffle-shuffling of a deck of cards, which arises in this manner from the
shuffle algebra.
The primary reason for constructing Hopf-power Markov chains, or for
rephrasing familiar chains through this lens, is that much information about
them comes simply from translating well-known facts on the underlying Hopf
algebra. For example, there is an explicit formula for the stationary
distribution (Theorem 4.5.1), and constructing quotient algebras show that
certain statistics on a Hopf-power Markov chain are themselves Markov chains
(Theorem 4.7.1). Perhaps the pinnacle is Theorem 2.5.1, a collection of
algorithms for a full left and right eigenbasis in many common cases where the
underlying Hopf algebra is commutative or cocommutative. This arises from a
cocktail of the Poincare-Birkhoff-Witt theorem, the Cartier-Milnor-Moore
theorem, Reutenauer's structure theory of the free Lie algebra, and Patras's
Eulerian idempotent theory.
Since Hopf-power Markov chains can exhibit very different behaviour depending
on the structure of the underlying Hopf algebra and its distinguished basis,
one must restrict attention to certain styles of Hopf algebras in order to
obtain stronger results. This thesis will focus respectively on a
free-commutative basis, which produces "independent breaking" chains, and a
cofree basis; there will be both general statements and in-depth examples.
|
1412.8221v2
|
2015-01-22
|
KELT-7b: A hot Jupiter transiting a bright V=8.54 rapidly rotating F-star
|
We report the discovery of KELT-7b, a transiting hot Jupiter with a mass of
$1.28 \pm 0.18$ MJ, radius of $1.53_{-0.047}^{+0.046}$ RJ, and an orbital
period of $2.7347749 \pm 0.0000039$ days. The bright host star (HD33643;
KELT-7) is an F-star with $V=8.54$, Teff $=6789_{-49}^{+50}$ K, [Fe/H]
$=0.139_{-0.081}^{+0.075}$, and $\log{g}=4.149 \pm 0.019$. It has a mass of
$1.535_{-0.054}^{+0.066}$ Msun, a radius of $1.732_{-0.045}^{+0.043}$ Rsun, and
is the fifth most massive, fifth hottest, and the ninth brightest star known to
host a transiting planet. It is also the brightest star around which KELT has
discovered a transiting planet. Thus, KELT-7b is an ideal target for detailed
characterization given its relatively low surface gravity, high equilibrium
temperature, and bright host star. The rapid rotation of the star ($73 \pm 0.5$
km/s) results in a Rossiter-McLaughlin effect with an unusually large amplitude
of several hundred m/s. We find that the orbit normal of the planet is likely
to be well-aligned with the stellar spin axis, with a projected spin-orbit
alignment of $\lambda=9.7 \pm 5.2$ degrees. This is currently the second most
rapidly rotating star to have a reflex signal (and thus mass determination) due
to a planetary companion measured.
|
1501.05565v2
|
2015-01-27
|
Tracing the Metal-Poor M31 Stellar Halo with Blue Horizontal Branch Stars
|
We have analyzed new HST/ACS and HST/WFC3 imaging in F475W and F814W of two
previously-unobserved fields along the M31 minor axis to confirm our previous
constraints on the shape of M31's inner stellar halo. Both of these new
datasets reach a depth of at least F814W$<$27 and clearly detect the blue
horizontal branch (BHB) of the field as a distinct feature of the
color-magnitude diagram. We measure the density of BHB stars and the ratio of
BHB to red giant branch stars in each field using identical techniques to our
previous work. We find excellent agreement with our previous measurement of a
power-law for the 2-D projected surface density with an index of
2.6$^{+0.3}_{-0.2}$ outside of 3 kpc, which flattens to $\alpha <$1.2 inside of
3 kpc. Our findings confirm our previous suggestion that the field BHB stars in
M31 are part of the halo population. However, the total halo profile is now
known to differ from this BHB profile, which suggests that we have isolated the
metal-poor component. This component appears to have an unbroken power-law
profile from 3-150 kpc but accounts for only about half of the total halo
stellar mass. Discrepancies between the BHB density profile and other
measurements of the inner halo are therefore likely due to the different
profile of the metal-rich halo component, which is not only steeper than the
profile of the met al-poor component, but also has a larger core radius. These
profile differences also help to explain the large ratio of BHB/RGB stars in
our observations.
|
1501.06631v1
|
2015-02-12
|
A clear age-velocity dispersion correlation in Andromeda's stellar disk
|
The stellar kinematics of galactic disks are key to constraining disk
formation and evolution processes. In this paper, for the first time, we
measure the stellar age-velocity dispersion correlation in the inner 20 kpc
(3.5 disk scale lengths) of M31 and show that it is dramatically different from
that in the Milky Way. We use optical Hubble Space Telescope/Advanced Camera
for Surveys photometry of 5800 individual stars from the Panchromatic Hubble
Andromeda Treasury (PHAT) survey and Keck/DEIMOS radial velocity measurements
of the same stars from the Spectroscopic and Photometric Landscape of
Andromeda's Stellar Halo (SPLASH) survey. We show that the average
line-of-sight velocity dispersion is a steadily increasing function of stellar
age exterior to R=10 kpc, increasing from 30 km/s for the young upper main
sequence stars to 90 km/s for the old red giant branch stars. This monotonic
increase implies that a continuous or recurring process contributed to the
evolution of the disk. Both the slope and normalization of the dispersion vs.
age relation are significantly larger than in the Milky Way, allowing for the
possibility that the disk of M31 has had a more violent history than the disk
of the Milky Way, more in line with cosmological predictions. We also find
evidence for an inhomogeneous distribution of stars from a second kinematical
component in addition to the dominant disk component. One of the largest and
hottest high-dispersion patches is present in all age bins, and may be the
signature of the end of the long bar.
|
1502.03820v1
|
2015-03-17
|
The Star Formation Histories of Local Group Dwarf Galaxies III. Characterizing Quenching in Low-Mass Galaxies
|
We explore the quenching of low-mass galaxies (10^4 < Mstar < 10^8 Msun) as a
function of lookback time using the star formation histories (SFHs) of 38 Local
Group dwarf galaxies. The SFHs were derived from analyzing color-magnitude
diagrams of resolved stellar populations in archival Hubble Space
Telescope/Wide Field Planetary Camera 2 imaging. We find: (1) Lower mass
galaxies quench earlier than higher mass galaxies; (2) Inside of virial radius
there is no correlation between a satellite's current proximity to a massive
host and its quenching epoch; (3) There are hints of systematic differences in
quenching times of M31 and Milky Way (MW) satellites, although the sample
sample size and uncertainties in the SFHs of M31 dwarfs prohibit definitive
conclusions. Combined with literature results, we qualitatively consider the
redshift evolution (z=0-1) of the quenched galaxy fraction over ~7 dex in
stellar mass (10^4 < Mstar < 10^11.5 Msun). The quenched fraction of all
galaxies generally increases toward the present, with both the lowest and
highest mass systems exhibiting the largest quenched fractions at all
redshifts. In contrast, galaxies between Mstar ~ 10^8-10^10 Msun have the
lowest quenched fractions. We suggest that such intermediate-mass galaxies are
the least efficient at quenching. Finally, we compare our quenching times with
predictions for infall times of low-mass galaxies associated with the MW. We
find that some of the lowest-mass satellites (e.g., CVn II, Leo IV) may have
been quenched before infall while higher mass satellites (e.g., Leo I, Fornax)
typically quench ~1-4 Gyr after infall.
|
1503.05195v1
|
2015-04-28
|
Nearly Optimal Deterministic Algorithm for Sparse Walsh-Hadamard Transform
|
For every fixed constant $\alpha > 0$, we design an algorithm for computing
the $k$-sparse Walsh-Hadamard transform of an $N$-dimensional vector $x \in
\mathbb{R}^N$ in time $k^{1+\alpha} (\log N)^{O(1)}$. Specifically, the
algorithm is given query access to $x$ and computes a $k$-sparse $\tilde{x} \in
\mathbb{R}^N$ satisfying $\|\tilde{x} - \hat{x}\|_1 \leq c \|\hat{x} -
H_k(\hat{x})\|_1$, for an absolute constant $c > 0$, where $\hat{x}$ is the
transform of $x$ and $H_k(\hat{x})$ is its best $k$-sparse approximation. Our
algorithm is fully deterministic and only uses non-adaptive queries to $x$
(i.e., all queries are determined and performed in parallel when the algorithm
starts).
An important technical tool that we use is a construction of nearly optimal
and linear lossless condensers which is a careful instantiation of the GUV
condenser (Guruswami, Umans, Vadhan, JACM 2009). Moreover, we design a
deterministic and non-adaptive $\ell_1/\ell_1$ compressed sensing scheme based
on general lossless condensers that is equipped with a fast reconstruction
algorithm running in time $k^{1+\alpha} (\log N)^{O(1)}$ (for the GUV-based
condenser) and is of independent interest. Our scheme significantly simplifies
and improves an earlier expander-based construction due to Berinde, Gilbert,
Indyk, Karloff, Strauss (Allerton 2008).
Our methods use linear lossless condensers in a black box fashion; therefore,
any future improvement on explicit constructions of such condensers would
immediately translate to improved parameters in our framework (potentially
leading to $k (\log N)^{O(1)}$ reconstruction time with a reduced exponent in
the poly-logarithmic factor, and eliminating the extra parameter $\alpha$).
Finally, by allowing the algorithm to use randomness, while still using
non-adaptive queries, the running time of the algorithm can be improved to
$\tilde{O}(k \log^3 N)$.
|
1504.07648v1
|
2015-06-26
|
Modeling and Simulation of Spin Transfer Torque Generated at Topological Insulator/Ferromagnetic Heterostructure
|
Topological Insulator (TI) has recently emerged as an attractive candidate
for possible application to spintronic circuits because of its strong spin
orbit coupling. TIs are unique materials that have an insulating bulk but
conducting surface states due to band inversion and these surface states are
protected by time reversal symmetry. In this paper, we propose a physics-based
spin dynamics simulation framework for TI/Ferromagnet (TI/FM) bilayer
heterostructures that is able to capture the electronic band structure of a TI
while calculating the electron and spin transport properties. Our model differs
from TI/FM models proposed in the literature in that it is able to account for
the 3D band structure of TIs and the effect of exchange coupling and external
magnetic field on the band structure. Our proposed approach uses 2D surface
Hamiltonian for TIs that includes all necessary features for spin transport
calculations so as to properly model the characteristics of a TI/FM
heterostructure. Using this Hamiltonian and appropriate parameters, we show
that the effect of quantum confinement and exchange coupling are successfully
captured in the calculated surface band structure compared with the quantum
well band diagram of a 3D TI, and matches well with experimental data reported
in the literature. We then show how this calibrated Hamiltonian is used with
the self-consistent non equilibrium Green's functions (NEGF) formalism to
determine the charge and spin transport in TI/FM bilayer heterostructures. Our
calculations agree well with experimental data and capture the unique features
of a TI/FM heterostructure such as high spin Hall angle, high spin conductivity
etc. Finally, we show how the results obtained from NEGF calculations may be
incorporated into the Landau-Lifshitz-Gilbert-Slonczewski (LLGS) formulation to
simulate the magnetization dynamics of an FM layer sitting on top of a TI.
|
1506.07932v2
|
2015-07-23
|
A Spectroscopic and Photometric Exploration of the C/M Ratio in the Disk of M31
|
We explore the ratio (C/M) of carbon-rich to oxygen-rich thermally pulsing
asymptotic giant branch(TP-AGB) stars in the disk of M31 using a combination of
moderate-resolution optical spectroscopy from the Spectroscopic Landscape of
Andromeda's Stellar Halo (SPLASH) survey and six-filter Hubble Space Telescope
photometry from the Panchromatic Hubble Andromeda Treasury (PHAT) survey.Carbon
stars were identified spectroscopically. Oxygen-rich M-stars were identifed
using three different photometric definitions designed to mimic, and thus
evaluate, selection techniques common in the literature. We calculate the C/M
ratio as a function of galactocentric radius, present-day gas-phase oxygen
abundance, stellar metallicity, age (via proxy defined as the ratio of TP-AGB
stars to red giant branch, RGB, stars), and mean star formation rate over the
last 400 Myr. We find statistically significant correlations between log(C/M)
and all parameters. These trends are consistent across different M-star
selection methods, though the fiducial values change. Of particular note is our
observed relationship between log(C/M) and stellar metallicity, which is fully
consistent with the trend seen across Local Group satellite galaxies. The fact
that this trend persists in stellar populations with very different star
formation histories indicates that the C/M ratio is governed by stellar
properties alone.
|
1507.06687v1
|
2015-07-24
|
Discovery and Validation of Kepler-452b: A 1.6-Re Super Earth Exoplanet in the Habitable Zone of a G2 Star
|
We report on the discovery and validation of Kepler-452b, a transiting planet
identified by a search through the 4 years of data collected by NASA's Kepler
Mission. This possibly rocky 1.63$^{+0.23}_{-0.20}$ R$_\oplus$ planet orbits
its G2 host star every 384.843$^{+0.007}_{0.012}$ days, the longest orbital
period for a small (R$_p$ < 2 R$_\oplus$) transiting exoplanet to date. The
likelihood that this planet has a rocky composition lies between 49% and 62%.
The star has an effective temperature of 5757$\pm$85 K and a log g of
4.32$\pm$0.09. At a mean orbital separation of 1.046$^{+0.019}_{-0.015}$ AU,
this small planet is well within the optimistic habitable zone of its star
(recent Venus/early Mars), experiencing only 10% more flux than Earth receives
from the Sun today, and slightly outside the conservative habitable zone
(runaway greenhouse/maximum greenhouse). The star is slightly larger and older
than the Sun, with a present radius of 1.11$^{+0.15}_{-0.09}$ R$_\odot$ and an
estimated age of 6 Gyr. Thus, Kepler-452b has likely always been in the
habitable zone and should remain there for another 3 Gyr.
|
1507.06723v1
|
2015-08-02
|
Optimal Radio Frequency Energy Harvesting with Limited Energy Arrival Knowledge
|
In this paper, we develop optimal policies for deciding when a wireless node
with radio frequency (RF) energy harvesting (EH) capabilities should try and
harvest ambient RF energy. While the idea of RF-EH is appealing, it is not
always beneficial to attempt to harvest energy; in environments where the
ambient energy is low, nodes could consume more energy being awake with their
harvesting circuits turned on than what they can extract from the ambient radio
signals; it is then better to enter a sleep mode until the ambient RF energy
increases. Towards this end, we consider a scenario with intermittent energy
arrivals and a wireless node that wakes up for a period of time (herein called
the time-slot) and harvests energy. If enough energy is harvested during the
time-slot, then the harvesting is successful and excess energy is stored;
however, if there does not exist enough energy the harvesting is unsuccessful
and energy is lost.
We assume that the ambient energy level is constant during the time-slot, and
changes at slot boundaries. The energy level dynamics are described by a
two-state Gilbert-Elliott Markov chain model, where the state of the Markov
chain can only be observed during the harvesting action, and not when in sleep
mode. Two scenarios are studied under this model. In the first scenario, we
assume that we have knowledge of the transition probabilities of the Markov
chain and formulate the problem as a Partially Observable Markov Decision
Process (POMDP), where we find a threshold-based optimal policy. In the second
scenario, we assume that we don't have any knowledge about these parameters and
formulate the problem as a Bayesian adaptive POMDP; to reduce the complexity of
the computations we also propose a heuristic posterior sampling algorithm. The
performance of our approaches is demonstrated via numerical examples.
|
1508.00285v1
|
2015-09-22
|
The Panchromatic Hubble Andromeda Treasury VIII: A Wide-Area, High-Resolution Map of Dust Extinction in M31
|
We map the distribution of dust in M31 at 25pc resolution, using stellar
photometry from the Panchromatic Hubble Andromeda Treasury. We develop a new
mapping technique that models the NIR color-magnitude diagram (CMD) of red
giant branch (RGB) stars. The model CMDs combine an unreddened foreground of
RGB stars with a reddened background population viewed through a log-normal
column density distribution of dust. Fits to the model constrain the median
extinction, the width of the extinction distribution, and the fraction of
reddened stars. The resulting extinction map has >4 times better resolution
than maps of dust emission, while providing a more direct measurement of the
dust column. There is superb morphological agreement between the new map and
maps of the extinction inferred from dust emission by Draine et al. 2014.
However, the widely-used Draine & Li (2007) dust models overpredict the
observed extinction by a factor of ~2.5, suggesting that M31's true dust mass
is lower and that dust grains are significantly more emissive than assumed in
Draine et al. (2014). The discrepancy we identify is consistent with similar
findings in the Milky Way by the Planck Collaboration (2015), but has a more
complex dependence on parameters from the Draine & Li (2007) dust models. We
also show that the discrepancy with the Draine et al. (2014) map is lowest
where the interstellar radiation field has a harder spectrum than average. We
discuss possible improvements to the CMD dust mapping technique, and explore
further applications.
|
1509.06988v1
|
2016-01-07
|
Detection of lensing substructure using ALMA observations of the dusty galaxy SDP.81
|
We study the abundance of substructure in the matter density near galaxies
using ALMA Science Verification observations of the strong lensing system
SDP.81. We present a method to measure the abundance of subhalos around
galaxies using interferometric observations of gravitational lenses. Using
simulated ALMA observations, we explore the effects of various systematics,
including antenna phase errors and source priors, and show how such errors may
be measured or marginalized. We apply our formalism to ALMA observations of
SDP.81. We find evidence for the presence of a $M=10^{8.96\pm 0.12} M_{\odot}$
subhalo near one of the images, with a significance of $6.9\sigma$ in a joint
fit to data from bands 6 and 7; the effect of the subhalo is also detected in
both bands individually. We also derive constraints on the abundance of dark
matter subhalos down to $M\sim 2\times 10^7 M_{\odot}$, pushing down to the
mass regime of the smallest detected satellites in the Local Group, where there
are significant discrepancies between the observed population of luminous
galaxies and predicted dark matter subhalos. We find hints of additional
substructure, warranting further study using the full SDP.81 dataset
(including, for example, the spectroscopic imaging of the lensed carbon
monoxide emission). We compare the results of this search to the predictions of
$\Lambda$CDM halos, and find that given current uncertainties in the host halo
properties of SDP.81, our measurements of substructure are consistent with
theoretical expectations. Observations of larger samples of gravitational
lenses with ALMA should be able to improve the constraints on the abundance of
galactic substructure.
|
1601.01388v1
|
2016-02-09
|
Isotropic at the Break? 3D Kinematics of Milky Way Halo Stars in the Foreground of M31
|
We present the line-of-sight (LOS) velocities for 13 distant main sequence
Milky Way halo stars with published proper motions. The proper motions were
measured using long baseline (5-7 years) multi-epoch HST/ACS photometry, and
the LOS velocities were extracted from deep (5-6 hour integrations) Keck
II/DEIMOS spectra. We estimate the parameters of the velocity ellipsoid of the
stellar halo using a Markov chain Monte Carlo ensembler sampler method. The
velocity second moments in the directions of the Galactic $(l,b,$ LOS)
coordinate system are $\langle v^2_l \rangle^{1/2} = 138^{+43}_{-26}$ km/s,
$\langle v^2_b \rangle^{1/2} = 88^{+28}_{-17}$ km/s, and $\langle
v^2_{\rm{LOS}} \rangle^{1/2} = 91^{+27}_{-14}$ km/s. We use these ellipsoid
parameters to constrain the velocity anisotropy of the stellar halo. Ours is
the first measurement of the anisotropy parameter $\beta$ using 3D kinematics
outside of the solar neighborhood. We find $\beta=-0.3^{+0.4}_{-0.9}$,
consistent with isotropy and lower than solar neighborhood $\beta$ measurements
by 2$\sigma$ ($\beta_{SN} \sim 0.5-0.7$). We identify two stars in our sample
that are likely members of the known TriAnd substructure, and excluding these
objects from our sample increases our estimate of the anisotropy to
$\beta=0.1^{+0.4}_{-1.0}$, which is still lower than solar neighborhood
measurements by $1\sigma$. The potential decrease in $\beta$ with
Galactocentric radius is inconsistent with theoretical predictions, though
consistent with recent observational studies, and may indicate the presence of
large, shell-type structure (or structures) at $r \sim 25$ kpc. The methods
described in this paper will be applied to a much larger sample of stars with
3D kinematics observed through the ongoing HALO7D program.
|
1602.03180v1
|
2016-06-22
|
Qatar Exoplanet Survey : Qatar-3b, Qatar-4b and Qatar-5b
|
We report the discovery of Qatar-3b, Qatar-4b, and Qatar-5b, three new
transiting planets identified by the Qatar Exoplanet Survey (QES). The three
planets belong to the hot Jupiter family, with orbital periods of
$P_{Q3b}$=2.50792 days, $P_{Q4b}$=1.80539 days, and $P_{Q5b}$=2.87923 days.
Follow-up spectroscopic observations reveal the masses of the planets to be
$M_{Q3b}$=4.31$\pm0.47$ $M_{\rm J}$, $M_{Q4b}$=6.10$ \pm0.54$ $M_{\rm J}$, and
$M_{Q5b}$ = 4.32$ \pm0.18$ $M_{\rm J}$, while model fits to the transit light
curves yield radii of $R_{Q3b}$ = 1.096$ \pm0.14$ $R_{\rm J}$, $R_{Q4b}$ =
1.135$ \pm0.11$ $R_{\rm J}$, and $R_{Q5b}$ = 1.107$ \pm0.064$ $R_{\rm J}$. The
host stars are low-mass main sequence stars with masses and radii $M_{Q3}$ =
1.145$ \pm0.064$ $M_{\odot}$, $M_{Q4}$ = 0.896$ \pm0.048$ $M_{\odot}$, $M_{Q5}$
= 1.128$ \pm0.056$ $M_{\odot}$ and $R_{Q3}$ = 1.272$ \pm0.14$ $R_{\odot}$,
$R_{Q4}$ = 0.849$\pm0.063$ $R_{\odot}$ and $R_{Q5}$ = 1.076$\pm0.051$
$R_{\odot}$ for Qatar-3, 4 and 5 respectively. The V magnitudes of the three
host stars are $V_{Q3}$=12.88, $V_{Q4}$=13.60, and $V_{Q5}$=12.82. All three
new planets can be classified as heavy hot Jupiters (M > 4 $M_{J}$).
|
1606.06882v5
|
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