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in the lambda cold dark matter hierarchical structure formation paradigm, a spiral galaxy's stellar halo is thought to have formed through the accretion of smaller dwarf galaxies onto the larger galaxy. the remnant stars from each accretion event trace out a different shape or morphology in the stellar halo, and as a whole these features are the substructure of the stellar halo. the amount and morphologies of substructural features helps determine the accretion history of the milky way, information which helps constrain models of the cosmological evolution of galaxies. the halo7d survey uses 7d observations (3d positions, 3d velocities, and metallicity) of halo stars to determine the milky way's accretion history; the purpose of our research is to design future halo7d observations by choosing optimal sky locations for keck observations and modelling observations of simulated galaxy halos to estimate how much information can be obtained from keck observations. based on the overlap of existing hubble space telescope archival images with appropriate depth, availability of reliable gaia astrometric data, sky coordinate ranges that ensure accessibility from the keck telescope at different times of the year, low interstellar dust extinction, and the lack of bright/extended galaxies or nebulae, we have identified 12 lines of sight (los) for follow up spectroscopic observations. utilizing bullock and johnston (2005) stellar halo simulations, we find that the distribution of observed stellar radial velocities is dependent on the time of accretion, with earlier accretion resulting in more homogeneous distributions across different los and late accretion resulting in greater differences across different los. these results show that the halo7d survey promises to constrain broad accretion trends in the milky way's history. further analysis is required to determine the dependence of these constraints on the specific subsample of stars observed, the location of the sun within the milky way, the number of stars per los, and the number of los. comparing the distributions of 3d positions, 3d velocities, and chemical abundances across the different los will give further insight on the robustness of the conclusions one can draw from the halo7d survey. wh, ap, js, and pg conducted their research under the auspices of the science internship program (sip) at the university of california santa cruz; pg was a fellow of the cal poly san luis obispo stem teacher and researcher (star) program. this research was funded in part by the national science foundation and nasa/stsci. | studying the accretion history of the milky way using mock observations of galaxy formation simulations |
a critical challenge to the cold dark matter (cdm) paradigm is that there are fewer satellites observed around the milky way than found in simulations of dark matter substructure. we show that there is a match between the observed satellite counts corrected by the detection efficiency of the sloan digital sky survey (for luminosities l >= 340 l⊙) and the number of luminous satellites predicted by cdm, assuming an empirical relation between stellar mass and halo mass. the ``missing satellites problem'', cast in terms of number counts, is thus solved, and implies that luminous satellites inhabit subhalos as small as 107 - 108 m⊙. the total number of milky way satellites depends sensitively on the spatial distribution of satellites. we also show that warm dark matter (wdm) models with a thermal relic mass smaller than 4 kev are robustly ruled out, and that limits of mwdm >= 8 kev from the milky way are probable in the near future. similarly stringent constraints can be placed on any dark matter model that leads to a suppression of the matter power spectrum on 107 m⊙ scales. measurements of completely dark halos below 108 m⊙, achievable with substructure lensing, are the next frontier for tests of cdm. ap acknowledges support from nsf under grant no. ast1615838. | there is no missing satellites problem |
we propose wfc3 uvis imaging of a new sample of 21 gravitationally lensed lyman-alpha emitting galaxies (laes) discovered within the million-galaxy spectroscopic database of the baryon oscillation spectroscopic survey (boss) of the third sloan digital sky survey (sdss-iii). these systems consist of massive galaxies at redshifts of approximately 0.55 strongly lensing laes at redshifts from 2 to 3. monte carlo simulation of our survey selection function indicates that these laes will all be strongly lensed and highly magnified. this represents the first statistically significant sample of strong galaxy-galaxy lenses with high-redshift laes as their background sources. viewed in rest-frame far-uv continuum light with wfc3 imaging, these laes have much more compact sizes than the lower-redshift lensed galaxies discovered by previous surveys (few 100 parsec versus few kiloparsec). this compactness makes lensed laes an ideal probe of dark substructures within the halos of the lensing galaxies, through observable perturbations to the predictions of smooth lensing mass models. we will use the constraints from substructure lensing in this new sample to make the most precise experimental measurement yet of the mass function and overall abundance of sub-halos in galaxy-scale parent halos, which will test a fundamental and robust prediction of the lambda-cdm model on subgalactic scales. we request zero proprietary period in order to make these important data available immediately to all scientists pursuing lensing measurements of dark-matter substructure. | quantifying cold dark matter substructure with a qualitatively new gravitational lens sample |
in this thesis, we harness the power of modern scientic computing to explore the formation and evolution of cosmological structure in a wide variety of astrophysical scenarios. we explore the nonlinear dynamics associated with the interplay between cold dark matter (cdm), baryons, ionizing radiation, and cosmic neutrinos, within regimes where analytic calculations necessarily fail. we begin by providing an overview of structure formation and its connections to the fields of study considered here: the epoch of reionization, galactic substructure evolution, and cosmic neutrinos. we then present a rigorous numerical convergence study of cosmological hydrodynamics simulations post-possessed with radiative transfer to study the impact of small-scale absorption systems within the intergalactic medium (igm) during the onset of reionization. we present converged statistics of the igm on smaller scales and earlier times than previously considered. moreover, we provide strict resolution limits for hydrodynamic simulations to properly resolve the unheated igm. next we study the infall and dynamical evolution of cdm halos in a galactic host. we find the behaviour of low-mass subhalos is qualitatively different than previously described for high-mass subhalos. in particular, the evolution of low-mass subhalos, with masses less than 0.1 per cent that of the host, is mainly driven by their concentration. this presents an opportunity to use concentration as a predictive indicator of substructure evolution. we finish this thesis with an investigation of a recently proposed method for constraining individual neutrino mass from cosmological observations. such a detection depends on the ability to reconstruct the cdm-neutrino relative velocity, which we show can be accomplished using linear transformations of an observed galaxy field. based on this, we perform the world's largest cosmological n-body simulation and present preliminary results for the observational prospects of cosmic neutrinos. | simulating the universe: nonlinear formation and evolution of cosmic structure |
the abundance of substructure within dark matter halos surrounding galaxies has been an area of intensive study for over a decade. the number of observed low-mass satellites of the milky way is about three orders of magnitude lower than what cold dark matter (cdm) simulations predict, an issue that is referred to as the "missing satellite problem". disagreement between the abundance of dark matter subhalos and the cdm predictions may lead to exciting insights into the micro-physics of dark matter particles. however, measuring the abundance of subhalos, which may be entirely composed of dark matter, requires a purely gravitational probe. i have simulated observations of gravitationally lensed sub-millimeter galaxies, showing that they can be used to detect dark matter subhalos in the lensing galaxies, using the gravitationally-induced distortions that they cause in the images of background sources. i have shown that by measuring the observed surface brightness correlations of lensed images we can detect the power spectrum of low-mass dark matter subhalos with high significance. in this talk, i will give an overview of an observational campaign that i'm leading to use alma and a newly discovered population of strongly lensed sub-millimeter galaxies to measure the mass function of dark matter subhalos with unprecedented accuracy. | mapping the small-scale structure of dark matter halos with strong gravitational lensing |
rr lyrae stars are powerful tracers of galactic structure, substructure (intact and disrupted dwarf satellite galaxies), accretion history, and dark matter content. the characteristic photometric variability pattern of rr lyrae stars makes it relatively easy to tell them apart from other stars (sample contaminants) and they are excellent standard candles (distance indicators). here we report the discovery of distant rr lyrae stars, including a handful of the most distant stars known in the milky way halo, with distances larger than 300 kpc. the data used in this study is taken in the u*g'i'z' bands with the canada-france-hawaii telescope as part of the next generation virgo cluster survey (ngvs). the ngvs covers 104 deg2 in the virgo direction with excellent imaging depth (10-sigma at g ~ 24.5 mag in a single exposure). we use a template light curve fitting method based on empirical sdss stripe 82 rr lyrae data to identify rr lyrae candidates from the sparse and unevenly sampled ngvs multiband data (~6-11 epochs in each filter across 4 years). when we test our detection algorithm on the ngvs light curves of 84 previously classified rr lyraes in pan-starrs1 (ps1) data, we achieve a recovery rate of 98.8%, with a period match at the 0.2% level. in total, we detect 289 rr lyrae candidates, with heliocentric distances from 20 kpc out to 320 kpc. compared with other rr lyrae surveys like ps1, hits and des, this ngvs study has smaller sky coverage and comparable cadences, but significantly better single-epoch photometric precision, consequently making our rr lyrae stars the most complete and robust sample at these galactocentric distances, with the best measured pulsation parameters. these newly discovered distant stars are an important addition to the few known tracers of the outermost regions of the milky way halo. yf is supported in part by the china scholarship council and the national science foundation. | discovery of the most distant milky way halo rr lyrae stars |
tidal streams are promising candidates for studying dark matter (dm) subhalos at the smallest scales predicted by cdm. studies have proposed that tidal streams can be perturbed by interactions with dm subhalos, causing over- and underdensities (known as "gaps") in the stellar density along the stream, which is otherwise presumed to be smoothly varying. we test this notion using two populations of dwarf galaxy streams from cosmological simulations of milky way-mass galaxies. the first group develops in the fire-2 environment (with dark substructure) in mw mass hosts, while the second group's stellar orbits are integrated in a smooth potential model of the same hosts. we compare the density profiles of the two populations using a gappiness statistic, , which is sensitive to a combination of the number of local minima and the density field curvature at each minimum. we find similar underdensities in tidal streams in both environments. additionally, the streams that develop in the smooth potential have similar (gappiness) to those that that develop in the environment with dark substructure. we hypothesize that other mechanisms could be responsible for gaps in tidal streams, such as reduced tidal stripping at progenitor apogee, or the epicyclic motion of stars. | effects of dark matter subhalos on tidal streams |
abell 1240 belongs to a rare class of merging galaxy clusters with symmetric double radio relics. in order to reconstruct the merging scenario of this interesting system, it is important to map out the dark matter distribution and identify substructures. we present the first weak-lensing analysis of abell 1240 with subaru observations after robustly addressing instrumental systematics. our weak-lensing analysis shows that the dark matter distribution of abell 1240 is bimodal and mimics the cluster galaxy distribution stretched from north to south and perpendicular to the radio relics. our mass reconstruction also detects the presence of abell 1237, which is located ~2 mpc to the south and is believed to be infalling onto abell 1240. we measure the cluster mass by assuming that the system follows an nfw profile and compare the results with dynamical estimates. in addition, we present our preliminary merging scenario combining the current weak-lensing results and other observations in the literature. | revealing substructures in abell 1240 with subaru weak gravitational lensing |
ngc1052-df2 and ngc1052-df4 are two puzzling ultra-diffuse galaxies: they have low velocity dispersions indicating little to no dark matter content, as well as overly luminous and large globular clusters compared to the milky way population. our group proposes a new model where these two galaxies formed jointly in the aftermath of a single bullet-cluster-like collision of dwarf galaxies. the gas separates from the dark matter in the collision and subsequent star formation leads to the formation of one or more dark-matter-free galaxies. the present-day line-of-sight distances and radial velocities of df2 and df4 are consistent with their joint formation in the aftermath of a single bullet-dwarf collision, around eight billion years ago. this is the first formation theory that can explain all the abnormal properties of both galaxies. in the ngc1052 group, we discover a linear substructure of seven to eleven large, low-luminosity dwarf galaxies which include df2 and df4. | a trail of dark matter-free galaxies from a bullet dwarf collision |
cosmological dark matter simulations predict that the central regions of halos have 1/r density cusps. baryonic processes and dark matter microphysics may resculpt these dark matter cusps, but these process are poorly understood and only weakly constrained by observations of a few low redshift halos. we have discovered a strong lensing cluster at redshift 1 that includes a bright central image like no other: the central image is resolved! this makes for a uniquely powerful probe of the central regions of a dark matter halo without the need for additional dynamical constraints. an nfw halo cannot reconstruct the data: modelling of our decam imaging with a double powerlaw density profile shows that the central slope must be much shallower than 1/r over at least the central 35 kpc. we propose to obtain high resolution imaging of this lens with wfc3. this data will be able to conclusively distinguish between a large core and an even larger shallow cusp. it will be the first such measurement at z = 1 and the most precise measurement of the central dm profile of a cluster ever made. the data will also allow for an investigation of dark matter substructures at z=1. when combined with cosmological hydrodynamical simulations these results will place new constraints on the range of baryonic processes and dm microphysics that can flatten the central density profiles of cluster dm halos over kiloparsec scales. | a unique probe of the dark matter distribution in a halo at z=1: a strong lens with a bright central image |
deap-3600 is a low-background, single-phase liquid argon (lar) direct detection experiment looking for nuclear recoils from wimp dark matter, operating 2 km underground at snolab (sudbury, canada). the detector consists of 3279 kg of lar contained in a spherical acrylic vessel. lar is an excellent scintillator, transparent to its own scintillation light. photomultiplier tubes detect the scintillation light, and pulse shape discrimination is applied to differentiate between nuclear recoils and electromagnetic interactions (the most abundant backgrounds, which predominantly come from the beta-decay of ar39). i will present an analysis of a 758 tonne-day exposure during 231 live day data set taken during the first year of operation. i will also discuss the current detector status, ongoing hardware upgrades, plans to improve the alpha-decays background discrimination in the detector neck region, recent updates of lar scintillation pulse shape analysis, and an update on wimp-search analysis including an approach using a non-relativistic effective field theory framework considering various possible substructures in the local dark matter halo to interpret the wimp results. on behalf of the deap3600 collaboration. | status of deap-3600 at snolab |
detecting hot gas in cluster outskirts is one of the major goals of our next-generation x-ray and sub-mm/sz facilities. it is important to understand how accretion and mergers shape clusters near and beyond the virial radius. even though cosmological simulations have been extensively used for these studies, idealized models have unique advantages of grasping underlying physics behind the complicated assembling history of galaxy clusters. in this talk, i will introduce our novel idealized models designed to explore the evolution of cluster outskirts, including both intracluster medium (icm) and dark matter (dm) halo. with our novel models, we built a theoretical framework – "runaway shock" scenario – describing the late evolution of merger shocks driven by infalling substructures (e.g., groups and small clusters) in the icm. the runaway merger shocks are considered as promising candidates for powering radio relics in the periphery of galaxy clusters. one of the beautiful real examples are shown in the coma cluster, observed by the up-to-date srg/erosita and lofar telescopes. the runaway shocks eventually overtake the accretion shock at the boundary of the icm and re-shape gaseous atmospheres in the outer cluster regions. various shocks and contact discontinuities are formed in this process. our simulations demonstrated that the radial location of the new icm boundary heavily depends on the parameter characterizing the smooth mass accretion rate of the system. in contrast, the evolution of the dm halo's boundary (i.e., splashback radius) depends on the total mass accretion rate contributed by both accretion and mergers. these results suggested that cluster mergers naturally explain the prominent radial offsets between the boundaries of the icm and dm halos in galaxy clusters long noticed in the cosmological simulations. | evolution of shocks and splashback boundaries in cluster outskirts |
a one-electron quantum cyclotron, quantum mon-demolition (qnd) detection, inhibited spontaneous emission, a split dilution refrigerator, and a self-shielded superconducting solenoid have been used to measure the electron's magnetic moment. this most precise determination of a property of an elementary particle, to 1.3 parts in 1013, was made to text the most precise prediction of the standard model of particle physics (sm). in the most precise confrontation of theory and measurement, the sm prediction agrees with what we measure to 1 part in 1012. bsm (beyond the sm) particles and electron substructure could make the measurement and prediction differ (like quark substructure shifts the proton moment). the measurement precision will allow a much better sm test once discrepant measurements of the fine structure constant are resolved. sm sectors involved include the dirac prediction, qed (quantum electrodynamics) through the 10th order with muon and tauon contributions, along with hadronic and weak interaction contributions. the quest to test the sm to find bsm physics is well motivated because the sm is known to be incomplete. no known cp violation mechanism is large enough to keep matter and antimatter produced in the big bang from annihilating as the universe cooled, dark matter has not been identified, and dark energy and inflation have no sm explanation. a new approach to detecting mev dark photons produced a 75 times lower limit that can be extended to a broad energy range with purpose-built apparatus. supported by the nsf, with detector development supported by the templeton foundation and trap cavity development support by the doe sqms center. | new measurement of the electron magnetic moment and a new dark photon limit |
stellar halo of a galaxy preserves most of the information and assembly history and it could play a crucial role to understand the hierarchical structure formation. the nonlinear nature of the hierarchical structure formation and the wide range of physical processes involved, demand full physics computational models but covering full dynamic range is restricted by extremely high computational cost. cosang (coupling semi-analytic and n-body galaxies) is a new approach in cosmological simulation, specifically designed for the stellar halo studies. in the first phase of cosang we have coupled a fast n-body model, gadget 3 with a semi-analytic model, sage. the semi-analytic model extracts baryonic information and cosang calculates the dynamical effects of baryons and feeds them back into the colissionless model. cosang is a self-consistent model and interacts with the simulation in all time-steps while it is running. we also use a comprehensive semi-analytic model which includes many physical processes with a possibility of exploring the parameter space more efficiently. lower computational cost of cosang will let us to explore the structure formation in much higher resolution and resolve much smaller substructures. we study the inner structure of the halo and the subhalo distribution in the first phase. using more realistic dynamics in cosang can provide more than dark matter only simulations using a newer, under development, particle tagging feature. in this phase cosang tags particles, according to their dynamical properties. the semi-analytic model provides more than full phase space information and we can paint particles and study metallicity, age, stellar mass of the stellar halo. this new approach could produce more realistic results in a higher resolution and could be calibrated by observations and hydrodynamic models. | cosmological stellar halo simulations with cosang |
strong gravitational lensing is a promising method for probing the nature and distribution of dark matter on sub-galactic scales. in addition to the subhalos of the main lens, dark matter halos along the line-of-sight between the observer and the source contribute significantly to the subtle perturbations to lensed images. however, dealing with these multiplane lensing effects is computationally expensive, so we propose a new approach called "effective multiplane gravitational lensing" to investigate the collective effect of line-of-sight halos and main-lens dark matter substructure on extended lensed arcs. in this approach, the lens mapping between the source and image planes can be fully characterized by two "effective" lensing potentials that encompass the entire structure of the deflection field, with the line-of-sight halos and main-lens substructure contributing differently to each potential. utilizing this new approach, we show that line-of-sight halos between the observer and the source imprint a distinct anisotropic signature characterized by a quadrupole moment of the two-point correlation function of the effective convergence field. we also show how this anisotropic signal has the potential to statistically distinguish the contribution of line-of-sight halo perturbations to lensing perturbations from that of main lens subhalos in a strongly lensed system, thereby significantly improving the constraint on dark matter from strong gravitational lensing. we further go over the effects of various dark matter models on these strong lensing anisotropies in more detail. | investigating dark matter microphysics with strong gravitational lensing anisotropies |
we develop two new methods to measure the structure of matter around the milky way using stellar tidal streams from disrupting dwarf galaxies and globular clusters. the dark matter halo of the milky way is expected to be triaxial and filled with substructure, but measurements of the shape and profile of dark matter around the galaxy are highly uncertain and often contradictory. we demonstrate that kinematic data from near-future surveys for stellar streams or shells produced by tidal disruption of stellar systems around the milky way will provide precise measures of the gravitational potential to test these predictions. we develop a probabilistic method for inferring the galactic potential with tidal streams based on the idea that the stream stars were once close in phase space and test this method on synthetic datasets generated from n-body simulations of satellite disruption with observational uncertainties chosen to mimic current and near-future surveys of various stars. we find that with just four well-measured stream stars, we can infer properties of a triaxial potential with precisions of order 5--7 percent. we then demonstrate that, if the milky way's dark matter halo is triaxial and is not fully integrable (as is expected), an appreciable fraction of orbits will be chaotic. we examine the influence of chaos on the phase-space morphology of cold tidal streams and show that streams even in weakly chaotic regions look very different from those in regular regions. we discuss the implications of this fact given that we see several long, thin streams in the galactic halo; our results suggest that long, cold streams around our galaxy must exist only on regular (or very nearly regular) orbits and potentially provide a map of the regular regions of the milky way potential. we then apply this understanding of stream formation along chaotic orbits to the interpretation of a newly-discovered, puzzling stellar stream near the galactic bulge. we conclude that the morphology of this stream is consistent with forming along chaotic orbits due to the presence of the time-dependent galactic bar. these results are encouraging for the eventual goal of using flexible, time-dependent potential models combined with larger data sets to unravel the detailed shape of the dark matter distribution around the milky way. | inferring the 3d gravitational field of the milky way with stellar streams |
in this paper, we investigated the influence of dark matter in galactic halos on the dynamics of galaxies in merger events, using n-body simulations. in the standard cosmological picture, large massive galaxies are formed by mergers of smaller ones. these mergers are very important for galactic dynamics and evolution. the largest portion of the galactic mass is the dark matter halo. dark matter has a very important role due to dynamical friction and the formation of observed substructures. here we investigated how properties of spherical dark matter halos and merger circumstances influence the formation of these morphological structures. | dark matter halos in galaxy mergers |
one outstanding question in cosmology is, what are the smallest galaxies that can form? the answer to this question can tell us much about galaxy formation, and even of the properties of dark matter itself. a candidate for the smallest galaxies that can form are the ultrafaint galaxies. the star formation of ultrafaints appears to have been shut off during the epoch of reionization, when radiation from the first stars ionized all the free hydrogen in the universe. this would imply ultrafaints should exist everywhere in the universe. however, we can only observe ultrafaints as satellites of the milky way, due to their low brightness. this will change with the next generation of telescopes such as the large synoptic survey telescope (lsst). the focus of this work is to predict the number of ultrafaints that should be seen with future surveys. to that end, we use the elvis suite, which contains 14 dark matter only simulations of local group like systems containing a milky way and andromeda-like galaxy and the substructure out to around 1 mpc of the barycenter. we mock observe the simulations in order to mimic current surveys such as the sloan digital sky survey (sdss), and the dark energy survey (des), and use the population of galaxies found by those surveys to project the population of dwarf galaxies out beyond the virial radius of either galaxy. this number will depend sensitively on the formation mechanism of ultrafaint dwarfs, and comparisons of future surveys to this work could help rule out certain formation scenarios. | the smallest galaxies in the universe: investigating the origins of ultra-faint galaxies |
relating observations of cluster galaxies or the gas content to the total mass of the underlying dark matter halos is a key challenge in the current cluster cosmology community. on the other hand, accurate measurement of hot and cold phase baryon covariance in clusters will offer important constraints on hydrodynamic models of cluster formation. this property covariance has been predicted by hydrodynamics simulations of wu et al. (2015). wu et al. (2015) predicts massive dark-matter halos are essentially ``closed box'' that retain all their gaseous and stellar matter, despite a diverse set of astrophysical disruptions happening inside them.we build a forward bayesian model to constrain the mass observable scaling relation and test ``closed box'' scenario through the property covariance, simultanously. we, then, present results of this method applied to multi-wavelength observations of clusters from the local cluster substructure survey (locuss). we find ~3σ evidence that at least one of the covariances between hot gaseous probes and stellar mass probes is negative. these results provide the first observational evidence in favor of the ``closed box'' nature of clusters at the high mass end. | closed box nature of galaxy clusters through multi-wavelength analysis |
like strings of lights wrapped around a tree, stellar streams trace out the gravitational potential of the milky way, as well as providing a snapshot of the process of accretion by which galaxies grow. in recent years, wide-area photometric surveys like sdss and des have revealed an abundance of stellar substructure in the milky way's halo. since 2018 s5 — an international collaboration launched in australia — has been working on the first complete spectroscopic census of southern hemisphere streams with 2df+aaomega on the aat. thus far, we have obtained spatial, kinematic and metallicity information for 12 streams in the des footprint, and we have expanded our program beyond the des footprint to cover more southern streams. the radial velocities and stellar metallicities from our spectra — together with proper motions from gaia edr3 — provide us with a unique dataset for understanding the milky way's halo. we are investigating stellar populations in the halo, the progenitors and formation processes of the streams, the accretion and interaction history of our galaxy, the mass and overall morphology of the milky way's gravitational potential, and ultimately the nature of dark matter. i will give an overview of our program and our first public data release, and what we have learned about the progenitors and orbits of the dozen streams, as well as additional recent science highlights from the s5 survey. | disentangling the holiday lights: tracing the milky way's accretion history and gravitational potential with stellar streams |
stars escaping globular clusters form thin, long and kinematically-cold tidal streams. in pristine conditions, these streams have nearly uniform density, however, new gaia observations of one such structure in the milky way halo have revealed a likely site of perturbation. the on-sky morphology suggests a recent, close encounter with a massive and dense perturber. known baryonic objects are unlikely perturbers based on their orbital properties, but observations permit a low-mass dark-matter subhalo as a plausible candidate. this observation opens up the possibility that detailed observations of streams could measure the mass spectrum of dark-matter substructures and even identify individual substructures and their orbits in the milky way halo. | dynamical evidence for a dark substructure in the milky way halo |
among the most fundamental predictions of lcdm cosmology is the existence of a large number of dark subhalos in and around galaxies, most of which should be entirely devoid of stars. confirming the existence of this substructure stands among the most important empirical challenges in modern cosmology. the goal of this proposal is to make robust lcdm predictions for dark halo substructure using high-resolution cosmological simulations that explicitly include the effects of baryons via a fast hybrid n-body method as well as full hydrodynamical runs. observational studies of strong gravitational lensing systems using hst and other instruments have begun to detect substructures with masses approaching those where halos may be entirely dark. there is real hope that similar studies can detect and quantify subhalos with masses 100 times smaller. moreover, the number of suitable lenses is expected to grow substantially in coming years. stellar streams around the milky way provide an independent means of detecting substructure. the number of cold stellar streams is too expected to grow as surveys like lsst come on line. while we are poised for phase-change in our ability to detect and quantify substructure, the current state of theoretical predictions is far behind. the vast majority of predictions include only dark matter and therefore neglect the destructive effect of central galaxies -- an effect that is know to reduce substructure significantly in the cores of halos, the very regions where observations are most sensitive. our simulations will overcome this deficiency and provide the predictions needed to accurately interpret observational results. | accurate predictions for dark matter substructure |
we introduce coupled semi-analytic n-body galaxies (cosang), a hybrid cosmological algorithm for stellar halo simulations. it combines a fast n-body model (gadget3) with a semi-analytic model (sage) to add baryonic potentials. this live interaction at each time-step can produce more accurate dynamics than pure n-body simulations. the semi-analytic model is also used to tag stellar populations onto dark matter particles, allowing us to "light up" the particles and trace the stellar mass, metallicity, age, and dynamics of the stellar halo. using cosang, we simulate a sample of zoom-in milky way mass halos at 105 m⊙ mass resolution. we apply cosang in three major studies: 1) dark matter kinematics: we study the substructures, global shapes, and twists of the dark matter halos in cosang compared to dark-matter-only simulations. the effect of the baryonic potential on the subhalo populations differs depending on the galaxy merger history. in the most complete regime, at small radius and high stellar mass, the simulated subhalo populations are compatible with those in the milky way and m31, though discrepancies remain. 2) stellar tracer kinematics: we estimate dark matter halo mass profiles with the spherical jeans equation using stellar halo populations produced through particle tagging. using stellar tracers results in a systematic underestimate of the halo mass due to the clumpiness of stellar halos in phase space, in contrast to random selections of dark matter particles, for which the jeans equation is unbiased. 3) stellar halo population analysis: we compare metallicity and age of the stellar haloes produced by cosang with the observed stellar halos. cosang produces a metallicity distribution with multiple peaks consistent with accretion histories and similar to observations. | connecting dark matter halos to stellar halos with coupling semi-analytic and n-body galaxies (cosang) |
one of the goals of theoretical galaxy formation is to use simulations combined with observations in order to constrain the fundamental physics that govern the formation of galaxies. of particular interest are physical processes that can impact the formation of small-scale structures. among the things we can potentially constrain with a combination of observations and simulations of small-scale structures are stellar feedback, reionization, and dark matter particle physics. we investigate several promising avenues to constrain fundamental physics with simulations of galaxy formation. first, we use the star formation histories (sfh) for local group dwarf galaxies in order to constrain high-redshift stellar mass functions (smfs). second, we look at the destruction of substructure around large galaxies, and how it may impact the detection of substructure via subhalo lensing. finally, we use full hydrodynamics simulations of local volume dwarf galaxies in order to investigate the impact of stellar feedback on the age distributions of stars within these galaxies. | exploring fundamental physics with small-scale structures |
the structure of dark matter halo is hierarchical. among them, small-scale structures in dark matter halo (so-called subhalos) can enhance dark matter annihilation signals. it is necessary to quantify boost factors by those subhalos to derive the property of dark matter with current/future gamma-ray observations. in order to derive the subhalo boost factors, calculations of halo structure covering more than 20 orders-of-magnitude in the halo mass up to a redshift of z ∼ 10 are required. this is beyond the capability of the current state-of-art cosmological n-body simulation which is a widely-adopted method to study the halo structure. in this talk, i introduce our analytical approach for the formalism of subhalo evolutions and the resultant boost factors. i show that the constraints on the annihilation cross-section obtained by isotropic γ-ray observations can be updated by several factors by taking the contribution from subhalos into account. | modeling evolution of dark matter substructure and annihilation boost |
in the prevailing lambda cold dark matter cosmology, galaxies grow continuously in mass through hierarchical assembly of smaller systems, and dark matter halos host substructures down to the resolution limit of the simulations. hierarchical evolution should occur at all scales, implying that the satellites of the milky way should also have companions. the recent discoveries of several ultra-faint dwarf (ufd) galaxy candidates in close proximity to the magellanic clouds (mcs) provide a unique opportunity to test and understand this process. in this context, i will present the results obtained for the ufds part of the milky way 6-d cosmology program (hst proposal 14734), whose stellar populations and color-magnitude diagrams (cmds) were analyzed to recover their detailed star formation histories (sfhs) with the synthetic cmd method, and understand the role that different parameters (stellar feedback, gas content, interaction state, environment) play in shaping their evolution. additionally, gaia dr2 proper motions were used to better constrain the possible dynamical association of these dwarfs with the mcs and retrace the history of formation and interaction of the magellanic system. | satellites of satellites: the power of combining hst photometry and gaia |
we present the results of alma observations of dust continuum emission and molecular rotational lines, including the alma compact array, toward a dense core mc27 (a.k.a. l1521f) in taurus, which is considered to be at very early stage of star formation. detailed column density distribution with a size scale from a few tens au to ∼10000 au scale are revealed by combining the alma data and the single-dish data. the high angular resolution observation at 0.87 mm reveals that a protostellar source, mms-1, is still not spatially resolved without gas association and a starless high-density core, mms-2, has substructures both in dust and molecular emission. the averaged radial column density distribution of the inner part (r . 3000 au) is nh2 ∼r −0.4 , clearly flatter than that of the outer part, ∼r -1.0 . we found the complex velocity/spatial structure obtained with previous alma observations is located inside the inner flatter region, which may reflect the dynamical status of the dense core. | stars are formed by mergers of dark matter planets |
simulations based on λcdm cosmology predict thousands of substructures under galactic scale have not been detected in the local universe. one hypothesis proposes that most of these substructures are dark for various astrophysical reasons. gravitational lensing provides a powerful alternative way to probe dark substructures in distant galaxies by detecting their gravitational perturbations and therefore provides insights into the nature of dark matter. lensed quasars with certain image configurations are especially promising for probing substructure abundance in lens galaxy halos. when the observed flux ratios of the lensed quasar images deviate from the smooth mass model predictions, these “flux-ratio anomalies” are considered to be the evidence of gravitational perturbations. while the standard analysis of flux-ratio anomalies assumes that substructures are the only cause of anomalies, we found that in two edge-on disk lenses, b1555+375 and b0712+472, their flux anomalies can be explained by including disk components into their mass models. our results bring up a concern with a potential bias in the previous analyses of flux-ratio anomalies. to further investigate the baryonic effects in flux-ratio anomalies, we create mock quasar lenses by selecting disk and elliptical galaxies in the illustris simulation. our analysis shows that baryon-induced flux anomalies can be found in all morphological types of lens galaxies. the baryonic effects increase the probability of finding lenses with strong anomalies by 8% in ellipticals and 10~20% in disk lenses, showing that the baryonic effects are unneglectable in the analysis. as future large-scale surveys are expected to bring numerous lensed quasar samples, further investigations on baryonic effects should be done in order to achieve precise constraints on dark matter in the future. | exploring a potential bias in dark matter investigations using strongly lensed quasars |
ever since the discovery of the first gravitational lens system by walsh, carswell and weymann in 1979, there has been tremendous excitement in the astronomy community about exploiting the power of gravitational lenses. lenses act as cosmic telescopes, magnifying the emission from sources in the background. this helps astronomers study objects that otherwise would not have been detected using current telescopes. lensing is also an excellent tool for studying cold dark matter substructure, the mass distribution of the lensing galaxies, the stellar processes in the lensing galaxies, and for constraining the hubble constant. one of the topics of ongoing debate in astronomy is the radio emission mechanism in radio quiet quasars. the two most likely processes responsible for the emission are star formation, and agn activity. understanding this emission mechanism is crucial, since it would give an insight into processes of star formation and black hole accretion, thus helping us understand galaxy formation and evolution in greater detail. even though radio quiet quasars form 90% of the quasar population, it is difficult to detect them because of their faint radio emission. with the combined power of lensing and current telescopes, lensed radio quiet quasars can be directly imaged and investigated. this thesis presents a study of 24 lensed radio quiet quasars. one of them is sdss j0924+0219, a lensed quasar that shows the most extreme flux anomaly in the optical ever seen in lensed systems. using vla and alma observations of this object, maps of the radio, submillimetre continuum, and co(5-4) molecular line were obtained. the intrinsic sources were reconstructed using lens modelling techniques. the radio and molecular gas emission were found to originate from the same region, thus suggesting that star formation in the host galaxy of the quasar is responsible for the radio emission. microlensing was found to be the reason behind the flux anomaly, a result that is consistent with previous studies of this object. the other 23 radio quiet quasars are a part of a larger sample that is being investigated using vla data to understand the nature of the radio emission in radio quiet quasars. the study shows mixed results, although more data is needed to make conclusive arguments. this thesis also presents the lofar long baseline calibrator survey, a survey whose aim is to find sources that could be used for calibration of data obtained using the international stations of the lofar telescope. a study of mg 0751+2716, a radio loud quasar observed using the international stations of lofar, is also presented. using the visibility data of the observation, lens modelling was undertaken to determine the intrinsic structure of the source. by comparing these results with a previous study at 27.4 ghz, it was found that the low frequency radio source is cospatial with the source at the higher frequency. | observations and modelling of gravitational lenses |
high-resolution studies of nearby stellar populations have served as a foundation for our quest to understand the nature of galaxies. today, studies of resolved stellar populations constrain fundamental relations -- such as the initial mass function of stars, the time scales of stellar evolution, the timing of mass loss and amount of energetic feedback, the color-magnitude relation and its dependency on age and metallicity, the stellar-dark matter connection in galaxy halos, and the build up of stellar populations over cosmic time -- that represent key ingredients in our prescription to interpret light from the universe and to measure the physical state of galaxies. more than in any other area of astrophysics, wfirst will yield a transformative impact in measuring and characterizing resolved stellar populations in the milky way and nearby galaxies. the proximity and level of detail that such populations need to be studied at directly map to all three pillars of wfirst capabilities - sensitivity from a 2.4 meter space based telescope, resolution from 0.1" pixels, and large 0.3 degree field of view from multiple detectors. our wfirst go science investigation team (f) will develop three wfirst (notional) go programs related to resolved stellar populations to fully stress wfirst's wide field instrument. the programs will include a survey of the milky way, a survey of nearby galaxy halos, and a survey of star-forming galaxies. specific science goals for each program will be validated through a wide range of observational data sets, simulations, and new algorithms. as an output of this study, our team will deliver optimized strategies and tools to maximize stellar population science with wfirst. this will include: new grids of ir-optimized stellar evolution and synthetic spectroscopic models; pipelines and algorithms for optimal data reduction at the wfirst sensitivity and pixel scale; wide field simulations of mw environments and galaxy halos; cosmological simulations of nearby galaxy halos matched to wfirst observations; strategies and automated algorithms to find substructure and dwarf galaxies in wfirst ir data sets; and documentation. our team will work closely with the wfirst science center to translate our notional programs into inputs that can help achieve readiness for wfirst science operations. this includes building full observing programs with target definitions, observing sequences, scheduling constraints, data processing needs, and calibration requirements. our team has been chosen carefully. team members are leading scientists in stellar population work that will be a core science theme for wfirst and are also involved in all large future astronomy projects that will operate in the wfirst era. the team is intentionally small, and each member will "own" significant science projects. the team will aggressively advocate for wfirst through innovative initiatives. the team is also diverse in geographical location, observers and theorists, and gender. | resolving the milky way and nearby galaxies with wfirst |
recent efforts to understand the dynamical history of the milky way by exploring the observational signatures of accreted substructure have yielded a number of interesting results. however, it is difficult to interpret the significance of the results in a cosmological context because the unique formation history of the milky way can lead to a distribution of substructure which significantly diverges from a typical galaxy of its mass. here, we attempt to contextualize the impact of the milky way's accretion history using the first suite of cosmological zoom-in simulations constrained to resemble the accretion history of the milky way (buch et al., in prep). the suite consists of 25 milky way mass host halos (1.0–1.8 × 1012 m☉) all of which were chosen to have an identified ge and lmc analog. we use detailed particle tracking to trace individual halos down to the simulation resolution limit (~108 m☉). this allows us to explore in detail the effect these recent mergers have on the anisotropy of the milky way's dark matter halo. in addition, combining particle tagging methods with stellar mass estimates from universemachine allows us to study the effects of merger history on phase-space clustering of disrupted substructure. these results can help to understand the robustness and biases of observational studies of phase-space clustering in the stellar halo. | understanding the impact of formation history on the dynamical distribution of substructure using the "milky way"-est simulation suite |
i present a brief overview of how stellar halos may be used to constrain the process of galaxy formation. in particular, streams and substructure in stellar halos trace merger events but can also be used to determine the mass distribution of the host galaxy and hence put constraints on the nature of dark matter. much of the focus of this contribution is on the milky way, but i also present an attempt to understand the kinematics of the globular cluster system of m31. | stellar halos and the link to galaxy formation |
we present results from follow-up spectroscopic observations of clustered cepheid candidates identified from k-band light curves towards the norma constellation (chakrabarti et al. 2015), as well as others that we have found more recently. the average radial velocity of these stars is ~ 200 km/s, which is large and distinct from that of the galaxy's stellar disk. these objects at l ~ -27 and b ~ -1 are therefore halo stars; using the period-luminosity relation of type i cepheids, they are at ~ 90 kpc. while the spectra do not have sufficient s/n to independently determine the metallicity and spectral type of the stars, there is a clear correspondence between the observed brackett series lines in these observations and in known type i cepheids. distances determined from the k-band period-luminosity relation (matsunaga et al. 2013) and the 3.6 μm period-luminosity relation (scowcroft et al. 2011) agree closely, and i-band observations have confirmed the periods of these sources. the extinction corrected j - ks colors of these sources are comparable to known type i cepheids (persson et al. 2004). the observed radial velocity of these stars agrees with predictions from dynamical models (chakrabarti & blitz 2009). if these stars are indeed members of the predicted dark-matter dominated dwarf galaxy that perturbed the outer hi disk of the milky way, this would represent the first application of galactoseismology. these observations also challenge models of the galactic halo. young cepheid variables are unexpected in models of the galactic halo, though star formation due to infall of gas-rich dwarf galaxies may well produce a small population of yet undiscovered cepheids in the outer halo. | a receding halo sub-structure towards norma |
tidal streams of globular clusters are ideal tracers of the galactic gravitational potential. compared to the few known, complex and diffuse dwarf-galaxy streams, they are kinematically cold, have thin morphologies and are abundant in the halo of the milky way. their coldness and thinness in combination with potential epicyclic substructure in the vicinity of the stream progenitor turns them into high-precision scales. with the example of palomar 5, we demonstrate how modeling of a globular cluster stream allows us to simultaneously measure the properties of the disrupting globular cluster, its orbital motion, and the gravitational potential of the milky way. | globular cluster streams as galactic high-precision scales |
the molecular clouds (mcs), which are observed as giant molecular clouds, isolated bok globules and/or infrared dark clouds, are the nurseries for forming stars and planets. observations show that the mcs are influenced by diverse forms of magnetic field lines. the magnetic field gradients can produce the ambipolar diffusion mechanism through the mcs. nejad-asghar (2019) showed that considering the heating due to ambipolar diffusion in the mcs, the local thermal balance leads to a local loosely constrained power-law relation between the pressure and density p ∝ ρ1+χ, where -0.4≤ χ≤ 0.05 depends on the functional form of the net cooling function. physically, the value of χ depends on the power of dependence of magnetic field to the density, and also on the value of the magnetic field gradient. for a strong magnetic field and/or a large field gradient, the value of χ decreases, and vice versa. the substructures through the mcs have complex morphologies from layers to filaments and semi-spheres. here, for simplicity, we use stratified layer approximation to investigate the effect of the non-isothermal parameter χ on the substructure of the mcs. the results show that considering the non-isothermal equation of state with smaller χ (i.e., stronger magnetic field and/or larger field gradient) transfers the magnetic field lines to the outer cloud regions, and hence decreases the density in the central regions of the cloud. we conclude that the stronger magnetic field and/or larger field gradient can disperse the density fluctuations through the mcs. | non-isothermal effects on the static equilibria of magnetized layers of molecular clouds |
collisions between galaxy clusters are a frequent scenario in the hierarchical model of structures. dissociative collisions provide an extreme environment of interaction between clusters of galaxies, where the properties of dark matter (dm) in relation to baryonic matter become evident. investigating dissociative scenarios allows a deeper understanding of the behavior and dynamics of baryonic and non-baryonic matter in this context of collision. abell 2034 (z = 0.114) is a bimodal system composed of a north and a south substructure, it has dissociative features observed in x-rays and gravitational lensing. using n -body hydrodynamic simulations, we present a theoretical study based on the dissociative collision of a2034, aiming to explore the effect that different relative concentrations between the clusters generate on the dynamics of the system. we investigated the relationship of the central density ratios with different levels of dissociation, where we analyzed nine models with different concentrations of the two components: intracluster gas and dm halo for each substructure. we found different degrees of dissociation that were quantified by the relative distance between the x-ray emission peak and the dark matter peaks. we found that the ratio of the gas central densities is more decisive than the ratio of dark matter central densities, in determining the level of dissociation for the parameters of this collision. | the role of gas and dark matter in the dissociative collision of galaxy cluster abell 2034 |
we use a catalogue of stellar binaries with wide separations (up to 1 pc) identified by the gaia satellite to constrain the presence of extended substructure within the milky way galaxy. heating of the binaries through repeated encounters with substructure results in a characteristic distribution of binary separations, allowing constraints to be placed independent of the formation mechanism of wide binaries. across a wide range of subhalo density profiles, we find that subhalos with masses ≳ 65 m⊙ and characteristic length scales similar to the separation of these wide binaries cannot make up 100% of the galaxy's dark matter. constraints weaken for subhalos with larger length scales and are dependent on their density profiles. for such large subhalos, higher central densities lead to stronger constraints. subhalos with density profiles similar to those expected from cold dark matter must be at least ~5,000 times denser than predicted by simulation to be constrained by the wide binary catalogue. | constraining dark matter substructure with gaia wide binaries |
in this work, we present updated results on an ongoing dark matter search using astrometric weak gravitational lensing. following the analysis in mondino et al. (2020), results are reported on a dark matter search using proper motions on stars in the magellanic clouds from gaia's most recent data release. additionally, we assess our method on star-star lensing, and introduce a parallax template method to aid in the dark matter search. finally, we provide preliminary sensitivity estimates for an acceleration-based search for compact dark matter structures. | astrometric weak lensing constraints on dark matter substructure with gaia edr3 |
fuzzy dark matter (fdm) has recently emerged as an interesting alternative model to the standard cold dark matter (cdm). in this model, dark matter consists of very light bosonic particles with quantum mechanical effects on galactic scales. since the small-scale behaviour of fdm is completely determined by the mass of the fdm particle, constraining the fdm axion mass remains a crucial test that would allow fdm to be verified or potentially excluded by existing observations. using the n-body code ax-gadget, we perform cosmological simulations of fdm that fully model the dynamical effects of the quantum potential throughout cosmic evolution. through the analysis of fdm volume and high-resolution zoom-in simulations of different fdm particle masses (m&~ ~ 10-23 - 10-21 ev), we study how fdm impacts the abundance of substructure and the inner density profiles of dark matter subhalos, respectively. for the first time, using our fdm volume simulations, we provide a fitting formula for the fdm-to-cdm subhalo mass function ratio as a function of the fdm axion mass. through comparison of our simulation results with observational inferences of the low-mass end of the subhalo mass function and the density profiles of dwarf galaxies surrounding the milky way, we will then place a constraint on the fdm axion mass. this material is based upon work supported by tamkeen under the new york university abu dhabi research institute grant cap3. | no catch-22 for fuzzy dark matter: testing substructure counts and core sizes via high resolution cosmological simulations |
we aim to understand the distribution of dark matter within both the milky way (mw) and external galaxies. in the mw, the dark matter distribution can be constrained now through direct acceleration measurements using pulsar timing and through indirect kinematic methods. my work aims to calibrate indirect methods of characterizing the galactic potential, such as from analysis of stellar streams, to direct acceleration measurements. in so doing, we are aiming to develop an "acceleration ladder", in analogy with the distance ladder. we combine potential constraints using action space clustering of stellar streams and local measurements of the potential from pulsar timing. our results indicate that the streams underestimate the local matter density, while the pulsars provide much stronger local constraints. meanwhile, stellar streams provide superior constraints on the global properties of the potential. the combination of these methods produces stronger overall constraints on the mw potential. for external galaxies, we focus on understanding the utility of gravitationally lensed supernovae (glsne). these systems contain information about the dark matter substructures within the lensing galaxy and along the line of sight. accurate time delay measurements can also produce strong independent constraints on the hubble constant. i have generated simulations of type ia and type ii-p glsne that include the effects of microlensing. our simulations indicate that accurate measurements of time delays can be recovered by using color curves in type ia glsne, which are largely insensitive to microlensing in the first ~30 days after the explosion. for type ii-p glsne, spectral observations can produce accurate time delays by measuring line velocities, especially feii, which decay following a power law. detections of these sources are predicted to be more numerous than type ia glsne, so measuring time delays in these systems is valuable. they can be used both for constraining the hubble constant and in the search for dark matter substructures. while supernovae are affected by microlensing, they are also sensitive to low mass dark matter substructure. this low mass sensitivity potentially allows for study of the relatively unexplored low mass end of the dark matter substructure distribution. | studying dark matter with pulsar timing, stellar streams and strong lensing |
understanding the observable astrophysical consequences of the fundamental properties of dark matter isessential for evaluating diverse observations by hst, from within streams and dwarf galaxies in or near themilky way, to effects on the formation and evolution of structure in cosmologically distant galaxies andclusters. we have developed statistical characterizations of the substructure within dark matter halos over awide range of masses and for a variety of assumptions about the physics of the dark matter particle, tosupplement and complement n-body simulations, which are challenged to reach the relevant mass scales. it ison these small scales where the interesting dark matter particle physics is expected to leave a detectable traceour objectives are: 1. provide the theoretical toolkit and suites of simulations that will be broadly useful for thehst community in analyzing observations in several key frameworks (local milky way galaxy-type structures;lens galaxies and clusters of galaxies at a range of cosmic epochs from z 0.1 through z 2); and 2. by takingthese sets of deliverables and running each realization through the gravitational lensing code of coi keeton,derive diagnostics that may distinguish between or rule out dark matter scenarios. | nonlinear evolution predictions for dark matter substructure, and predictions for gravitational lensing probes |
cosmological observables, from the cmb anisotropy to the census of galaxies in the local universe, offer the most direct and broad tests for the nature of dark matter, including a number of scenarios that are challenging or even impossible to test in a laboratory setting. i will review the status of the recent early-universe and late-universe searches for the identity of dark matter, summarizing the best current limits on scattering between dark matter and baryons, the non-thermal production mechanisms for sterile neutrinos, and mass bounds on thermal-relic dark matter. i will highlight the interplay between complementary probes of dark matter physics, focusing especially on the substructure in the milky way and its potential to resolve cosmological tensions. finally, i will discuss the prospects for unveiling the physics of dark matter in the coming decade. | dark matter physics from near-field cosmology |
we have recently pointed out that flattening rotation curves v(r) are naturally explained by elongated (prolate) dark matter (dm) distributions, and provided competitive fits to the sparc database. to further probe the geometry of the halo one needs out-of-plane observables. stellar streams in the milky way, poetically analogous to airplane contrails, but caused by tidal dispersion of massive substructures such as satellite dwarf galaxies, would lie on a plane (consistently with angular momentum conservation) should the gravitational field of the dm halo be spherically symmetric. entire orbits are seldom available because their periods are commensurable with hubble time, with streams often presenting themselves as short segments. therefore, the systematic study of the stellar stream torsion, a local observable that measures the deviation from planarity in differential curve geometry, provides sensitivity to aspherical dm distributions and ensures the use of even short streams. supported by grants micinn: pid2019-108655gb-i00/aei/10.13039/501100011033, pid2019-106080gb-c21(spain); ucm research group 910309 and the iparcos institute. | the torsion of stellar streams due to a nonspherical dark matter halo |
the analysis of optical images of galaxy-galaxy strong gravitational lensing systems can provide important information about the distribution of dark matter at small scales. however, the modeling and statistical analysis of these images is extraordinarily complex, bringing together source image and main lens reconstruction, hyper-parameter optimization, and the marginalization over small-scale structure realizations. we present here a new analysis pipeline that tackles these diverse challenges by bringing together many recent machine learning developments in one coherent approach, including variational inference, gaussian processes, differentiable probabilistic programming, and neural likelihood-to-evidence ratio estimation. our pipeline enables: (a) fast reconstruction of the source image and lens mass distribution, (b) variational estimation of uncertainties, (c) efficient optimization of source regularization and other hyperparameters, and (d) marginalization over stochastic model components like the distribution of substructure. we present here preliminary results that demonstrate the validity of our approach. | targeted likelihood-free inference of dark matter substructure in strongly-lensed galaxies |
we study the dynamical states of the 30 most massive galaxy clusters in the tng100 simulation at redshift z = 0 using three types of tracers: stars, dark matter particles and satellite galaxies. if the massive galaxy cluster is spherically symmetric and relaxed, we can obtain the underlying total mass distribution accurately from its dynamical tracers using the spherical jeans equations. although the three tracers of clusters have very different number densities, velocity dispersions and anisotropies, they still trace the same total mass profile. we obtain the total mass profiles of clusters using these tracers separately and compare them with the true mass distributions. we find that: (1) the kinematics of dark matter trace the total mass of all clusters well and the mass inferred from dark matter are generally consistent with the true mass profiles with relative deviations smaller than ∼ 25% at all radii; (2) stars in ∼ 60% massive clusters are approaching equilibrium and the total mass of these clusters inferred from stars have relative deviations smaller than ∼ 50% at all radii. stellar substructures are rich and the mass inferred from stars tend to be over-estimated in the inner region; and (3) satellite galaxies are unrelaxed in the inner region and become more relaxed as the radius increases. the total mass inferred from satellites are under-estimated in all regions. | the relaxation of galaxy clusters at redshift z = 0 in illustristng simulation |
a key prediction of cold dark matter (cdm) cosmology is a scale-free hierarchy of bound structure, from the largest galaxy clusters down to earth-mass microhalos. when partnered with the weakly interacting massive particle (wimp) high-energy physics description of cdm, this simple cosmological model makes accurate predictions for the abundance of dark matter in the universe. it successfully predicts dark matter clustering on large scales. however, wimps have never been detected in the lab, and there are persistent hints that small-scale structure is underabundant compared to cdm predictions. a direct observation of small dark matter halos would either lend strong support to the wimp/cdm paradigm, or conclusively kill the dominant phenomenological model of dark matter. gravitational lensing is a most promising tool for discovering small (<10^9 solar mass) halos outside of the milky way. until recently, the primary barrier to progress was observational; few gravitational lens systems were known and amenable to substructure lensing studies. but as many thousands of new lenses are found in wide-field surveys, innovative new methods to maximally exploit lenses mature, and with the essential follow-up capability of jwst and wfirst, we will for the first time be operating in a data-rich environment for substructure lensing. the missing ingredient to turn these data into a definitive test of cdm or non-cdm models is a comprehensive theoretical framework linking dark-matter microphysics to observables, marginalizing over uncertainties in galaxy evolution physics, host properties, and line-of-sight structure. while several basic estimates exist in the literature, there are no dedicated simulations that include all these effects for lens-mass systems. our proposed theoretical program is essential for turning the incredible potential of the forthcoming substructure lensing data into an unprecedented constraint on dark matter models. we propose to forecast constraints on dark matter physics from substructure lensing observations with jwst and wfirst, based on the theoretical framework that we create as part of this proposal. we will construct a set of probabilistic models and likelihood functions that are based on the semi-analytic model we will develop. semianalytic models are tuned to faithfully match n-body simulations, but run in a tiny fraction of the time. their speed enables scans through dark matter model parameter space; they can generate ensembles of systems matching observed hosts; and can explore and marginalize over the uncertainties in galaxy evolution feedback on small-scale structure. we will run carefully staged simulations in order to better parameterize evolutionary effects on dark matter halos, including the effects of baryons on the observations, and to test our semi-analytic model. the final product of this work will be a set of proposed jwst and wfirst follow-up campaigns that will maximize dark matter science as a function of observation time, with the expected dark matter constraints and theoretical uncertainties well quantified. we will make the semi-analytic model and our likelihood functions public, to serve the whole dark matter astrophysics community. with this work, we will finally realize the potential of substructure lensing to test the most fundamental prediction of the cdm paradigm. | seeing in the dark: revealing dark matter microphysics with substructure lensing |
taking secondary particles produced from dark matter (dm) annihilation process to the origin of the extended diffuse radio emission observed in galaxy clusters, we studied both their morphology and radio spectral profile using simulated coma like galaxy clusters. we have considered a neutralino annihilation channel dominated by $b\overline b $ species with a branching ratio of 1 and neutralino mass of 35 gev with annihilation cross-section of 1×10-26 cm3 s-1. the radio emission maps produced for the two simulated galaxy clusters which are based on the music simulation of galaxy clusters (music) dataset reveal the observed radio halo morphology showing radio emission both from the central regions of the cluster and substructures lying out off cluster centre. the flux density curve is in a good agreement for ν ≤ 2 ghz with the obsevational values for the coma cluster of galaxies showing a small deviation at higher frequencies. | non-thermal radio emission from dark matter annihilation processes in simulated coma like galaxy clusters |
self-interacting dark matter (sidm) predicts that dark matter halos experience core-collapse, a process where the halo's inner region rapidly increases in density and decreases in size. because of the short timescales and high densities involved, n-body simulations of the process are numerically challenging. we perform convergence tests of core-collapsing sidm halos across a range of halo concentrations and sidm cross-sections, and quantify potential numerical issues. due to the extreme nature of core-collapse, a convergence study of this level of detail has not been performed previously. we determine the necessary constraints to avoid these numerical issues and discuss how to diagnose them. using these newly tested simulations, we probe the physics of sidm, including calibration of the gravothermal fluid model and possible dark matter constraints from substructure lensing. | convergence tests and applications of sidm core-collapse simulations |
smaller substructures merged at different times to form the present-day milky way, and evidence for accretion has been discovered by various astronomical surveys. among all, the gaia collaboration released radial velocity measurements of 33 million stars in the milky way in data release 3 (dr3), a factor of 5 larger than the previous data release. here we use machine learning to distinguish between stars born within the milky way, or in situ stars, and stars that merged with it, called accreted stars. to build such a catalog, we trained our models on the synthetic survey ananke, based on the fire cosmological zoom-in hydrodynamic simulations of milky way-mass galaxies. the technique of "transfer learning" ensures the simulations' properties do not affect our results from the dr3 data. algorithms are first trained using 5d data and then applied to the 6d data in phase space with additional radial velocities. the models are then further refined by training on ananke dr3 following the 5d to 6d regime before being applied to dr3. the resulting accretion catalog will be made publicly accessible and can facilitate further analyses of the milky way's merger history and dark matter distribution. | using machine learning to catalog accreted stars in gaia esa dr3 survey |
the large-scale structure (lss) of the universe provides scientists with one of the best laboratories for studying lambda cold dark matter (lambdacdm) cosmology. especially at high redshift, we see increased rates of galaxy cluster and galaxy merging in lss relative to the field, which is useful for studying the hierarchical merging predicted by lambdacdm. the largest identified bound structures, superclusters, have not yet virialized. despite the wide range of dynamical states of their constituent galaxies, groups, and clusters, they are all still actively evolving, providing an ideal laboratory in which to study cluster and galaxy evolution. in this dissertation, i present original research on several aspects of lss and lambdacdm cosmology. three separate studies are included, each one focusing on a different aspect. in the first study, we use x-ray and optical observations from nine galaxy clusters at high redshift, some embedded in larger structures and some isolated, to study their evolutionary states. we extract x-ray gas temperatures and luminosities as well as optical velocity dispersions. these cluster properties are compared using low-redshift scaling relations. in addition, we employ several tests of substructure, using velocity histograms, dressler-shectman tests, and centroiding offsets. we conclude that two clusters out of our sample are most likely unrelaxed, and find support for deviations from self-similarity in the redshift evolution of the lx-t relation. our numerous complementary tests of the evolutionary state of clusters suggest potential under-estimations of systematic error in studies employing only a single such test. in the second study, we use multi-band imaging and spectroscopy to study active galactic nuclei (agn) in high-redshift lss. the agn were identified using x-ray imaging and matched to optical catalogs that contained spectroscopic redshifts to identify members of the structures. agn host galaxies tended to be associated with the transitional `green valley' on a color-magnitude diagram. spectral analysis of the agn hosts showed that the average host galaxy had either on-going or recent star formation, and was younger than the average galaxy, across all lss in our sample. we further subdivided our sample in two based on the average evolutionary state of the lss. the agn in the more evolved structures had lower x-ray luminosities and longer times since last starburst. these results provide some evidence for merger-based agn triggering, although other mechanisms, and possibly more than one, could be responsible. in the third study, we probed lambdacdm cosmology from a different angle. an important part of the model is the cosmological parameters that define our universe. as such, probes that can more accurately and precisely measure these parameters, such as h0 and the dark energy equation of state, w, can allow us to more closely inspect the model. strongly-lensed quasars provide one such probe, and we sought to perform the first step in using them for cosmological inference, which is to measure the time delays between strongly lensed images. we performed radio monitoring campaigns on six strongly lensed quasars using the very large array. lightcurves were extracted for each lensed image and analyzed for intrinsic variability. two lensed quasars showed strong time variations, but the variations were linear in time, preventing precise time delay measurements due to a degeneracy with the magnifications. these results suggest most of the systems should be targeted for followup monitoring, and we estimate that time delays can be measured for the most variable systems with precision of 0.5 to 3.5 days with two more seasons of monitoring. in a joint fit with previously studied systems, these measurements could tighten constraints on h 0 by up to ~1.4. | cosmological studies with galaxy clusters, active galactic nuclei, and strongly lensed quasars |
when a quasar is gravitationally lensed by a galaxy, its multiple images show light-curves that are offset by awell defined time delay, which depends on the mass profile of the lens and on cosmological distances to the lens and the source. by measuring the time-delay and accurately modelling the deflector's mass profile, this provides one-step measurements of cosmological distances to objects at redshift $z\sim1,$ whence the cosmological parameters (primarily $h_0$). one can turn this argument around and learn about galaxies instead, or even perform a joint (and less biased) inference. the joint modelling of the lens, the source structure and time-variability implies that the dm halos of lens galaxies at z~0.4-1 and the source properties of quasars and their hosts at z~1-2are inferred, besides information on cosmology that is complementary to other low-redshift probes such as sn ia and bao.a large (n~100) sample of lensed quasars will be transformative in this sense, as these systems are rare on the sky.i will describe our strides[*] searches in the dark energy survey, aiming at 120 previously unknown lensed quasars brighter than i=21. candidates have been selected with a variety of data mining techniques and flagged for follow-up (on spectroscopy, high-resolution imaging and lightcurve variability), which will take place in the following months. i will also cover recent modelling development of already monitored lenses within our collaboration, including a sharp multi-band reconstruction of the sources and use of stellar kinematics to ensure unbiased uncertainties on the lens mass profiles.this will lead to: (i) percent-level uncertainties on cosmological parameters(ii) insight on the coevolution of quasars and their host galaxies throughout cosmic time, up to z~2(iii) a quantative description of dark matter density profiles and the substructure content in massive galaxies up to z~1.[*] strides.physics.ucsb.edu | strides: galaxy evolution over cosmic time from new samples of gravitationally lensed quasars |
stellar halos give insight to the initial conditions that existed when a host galaxy first formed and provide details on disrupted satellites by looking at the different stellar populations. an algorithm that is computationally inexpensive compared to hydrodynamic simulations is necessary in order to theoretically study the structure and formation of galactic stellar halos in sufficient detail to probe substructure. currently being developed is cosang (coupled semi-analytic/n-body galaxies), a new computational method that will couple pure dark matter n-body simulations with a semi-analytic model. at each timestep, results from the n-body simulation will feed into the semi-analytic code, whose results will feed back into the n-body code making the evolution of the dark matter and baryonic matter dependent on one another. cosang will require much less computing power than hydrodynamical simulations, and will enable a variety of galaxy formation science, including analysis of stellar populations, halo merging, satellite accretion, supermassive black holes, and indirect and direct dark matter detection. | coupling semi-analytic models and n-body simulations: a new way of making galaxies and stellar halos |
we apply the jeans equation to identify subhalos in high-resolution numerical simulations that are potentially consistent with observed properties of milky way dwarf spheroidal (dsph) galaxies. we investigate the effects found by several recent studies that show changes to the dynamical properties of galactic substructure: the properties of dark matter (dm), and galaxy formation physics. for those subhalos that we found to provide reasonable fits to the high-quality photometric and kinematic data, they exhibit different properties at present time (e.g. maximum-circular velocity, mass, stellar distribution) and different formation history among the different scenarios that we consider here. thus those different scenarios provide unique signatures that can be further tested by future surveys. we also examine the possibility of matching the observed dsph luminosity with predicted kinematic properties simultaneously by abundance matching methods and hydrodynamical simulations. these results show that each mw dsphs has had a different star formation history, and these must be understood in concert with dm properties and galaxy formation to better understand the puzzles on sub-galactic scales. | the kinematics of milky way satellites as a test of dark matter models |
despite the success of the lambdacdm model at describing the universe on large scales, the model's predictions are at odds with observations on sub-galactic scales. in particular, the model predicts that a milky-way-sized dark matter halo should contain substructures in the form of low mass subhalos. however, there has yet to be convincing evidence that these subhalos exist. one promising method to probe the abundance of subhalos is by their dynamical effects on tidal streams, which are elongated remnants of tidally disrupted stellar systems as they orbit around the host galaxy. previous theoretical studies have shown that subhalos can induce longitudinal density variations (or "gaps") along streams. although gaps have indeed been observed in some streams, it remains unclear whether those gaps correspond to those expected from subhalos. previous simulations used idealized models, which did not account for realistic effects that may also induce gaps along streams. in this thesis we investigate a number of these effects. in the first chapter, we simulate streams with self-gravity, in which we successfully model the gaps caused by epicyclic overdensities, intrinsic to the orbit of a stream, rather than subhalo perturbations. we show that the two kinds of gaps are distinguishable in their length distributions. in the second chapter, we simulate self-gravitating streams inside a realistic halo that is constructed from a high-resolution cosmological simulation. we find that streams with similar orbital apo- and pericenters can have very different morphologies which cannot be modelled in idealized halos. in the third chapter, we simulate a collection of self-gravitating streams inside the same realistic halo in order to quantify the difference between using an idealized halo and a realistic halo. we show that the shape of the halo has a large effect on the dispersal of streams. nevertheless, we find that many of those streams still remain cold and thin. this suggests that upcoming stellar surveys are likely to discover more streams that are suitable for probing the abundance of subhalos. | dynamical signatures of dark matter substructures on tidal streams |
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 = 108.96±0.12m subhalo near one of the images, with a significance of 6.9σ 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 ∼ 2×107m, 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 λcdm 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. | commentary on hezaveh et al. 2016 titled "best to forget lcdmhc" |
in this work, different aspects of the high-energy radiation are looked at considering the lhc scenario. an event-shape variable and several jet substructure observables are studied with the mote carlo event simulators at the 13 tev center of mass energy scale to mimic the current lhc environment. the event-shape and the jet substructure observables are chosen such that they are not only sensitive to the different aspects of the high energy radiation measurement but also exhibit promising features to distinguish the possible existence of new physics that considers a dark matter candidate decaying into semi-visible jet. it is verified that the observables exhibit significant sensitivities to disentangle two jets to multi-jet radiations, presence of a final state and initial state radiations, presence of a large amount of missing transverse energy as a strong indication of the possible existence of a dark matter as well as couple of promising features of a semi-visible jet are explored. | a simulation study of the soft and hard radiations using jets at the lhc |
in the currently favored cold dark matter (cdm) model, dark matter is concentrated into gravitationally bound clumps known as halos and subhalos. dark matter is only known to interact gravitationally, making subhalos difficult to detect unless they contain baryons. stellar streams, extended tails of stars formed from the tidal disruption of globular clusters or low-mass galaxies, are extremely sensitive to changes in the gravitational potentials in which they orbit. thus, stellar streams are ideal candidates for studying dark matter substructures. this work focuses on two milky way streams: gd-1 and palomar 5. they show variations in density and width distributions that are best explained by gravitational perturbations. we analyzed sets of simulated stream data that contain varying amounts of subhalo populations ranging from no subhalos to three times the expected number of subhalos in λcdm. we compute the stellar density, width, velocity dispersion, and stream track as a function of position along the streams to determine the effect of subhalo interactions (flybys) on the streams. in looking at these kinematic effects on stellar streams, we observe that subhalo interactions induce density gaps, higher velocity dispersions, and wider stream widths. quantifying these gravitational effects with additional simulations can allow us to better identify stream perturbations to ultimately constrain the subhalo mass function. | measuring dark matter with stellar streams |
the fornax cluster provides an unparalleled opportunity to investigate the formation and evolution of earlytype galaxies in a dense environment. deep-field photometric surveys of the fornax cluster, conducted using omegacam and vircam/vista, have provided excellent data for studying the fornax environment. a step further, we are using these photometric observations to conduct the fornax cluster spectroscopic survey (fvss). as part of our fvss observations, we used spectroscopic data obtained from the visible multi-object spectrograph at the very large telescope (vlt/vimos) to kinematically characterize the globular cluster candidates detected photometrically in the cluster's core. combined with previous literature, we have compiled the largest spectroscopic sample of globular clusters (gcs) in the fornax environment, consisting of 2300 objects. we used the radial velocity catalogue of gcs to perform dynamical mass modeling of brightest cluster galaxy ngc1399 up to 200 kpc (∼ 6 𝑟𝑒𝑓𝑓) and examined the impact of intra-cluster gcs on the mass-modelling results. in this talk, i will present the kinematics of the gcs of the fornax cluster core region and will discuss the distribution of baryonic and dark matter around ngc1399. i will discuss the kinematic substructure of the intracluster gcs, which indicates their accreted nature. i will also present the ongoing fvss observations conducted with flames and fors2 to understand the mass assembly of the fornax cluster. youtube talk link : https://www.youtube.com/watch?v=d5itpeqldnw | utilizing eso wide field imaging surveys to learn the mass-assmebly of fornax cluster |
the standard cdm model is believed to have problems on small - galactic and sub-galactic - scales, namely the substructure problem (ssp), too-big-to-fail (tbtf) problem and core-cusp problem (ccp). recently, we've shown that a two-component (e.g., flavor-mixed) dark matter (2cdm) model can resolve all these problems altogether via particle elastic collisions (a lá sidm) and particle mass conversions. 2cdm does not have the early-universe problem faced by some alternative multicomponent models, yet it resolves the ssp and tbtf that sidm cannot do. in the new suite of simulations reported here, we explored the velocity dependence of cross-sections of scattering and conversion, σ(v) =σ0va , where σ0 and a are constants. we found that (i) 2cdm predictions are robust; (ii) many specific 2cdm models are consistent with available observed velocity functions in a wide range of σ0, i.e., between 1-0.1 cm2/g, and even down to 0.01 cm2/g for some; (iii) core sizes in dwarfs and clusters further constrain viable models. these models are to be explored in simulations with baryons, star formation and baryonic feedback. partially supported by doe grant de-sc0016368. | dark-matter-only simulations of the 2cdm model with σ(v) as a solution to the cdm small-scale problems |
we show how the higgs boson exchange processes may indicate the occurence of special higgs boson structures (substructures or peculiar interactions with dark matter) from a possible modification of the s-dependence of their cross section. we illustrate the simplest example with the $\mu^+\mu^-\to f\bar f$ process. | higgs boson structure from the shape of the cross section in exchange processes |
context: galaxy clusters are the most massive gravitationally bound structures in the universe and are formed through the process of hierarchical clustering, in which smaller systems undergo a series of mergers to form ever larger clusters. because of the masses involved, mergers between these giants provide a unique laboratory for observing many interesting astrophysical processes. these merging systems also act as large dark matter colliders, because the dark matter halos of the clusters involved pass through each other during of the merger. this offers us a means to observe if dark matter-dark matter collisions result in momentum exchange beyond what occurs from gravity alone. such observations can help us to unravel some of the mysteries behind dark matter, such as does it interact with itself through mechanisms beyond gravity, and how strong are those interactions. answers to questions like these are what will eventually allow us to discover what dark matter really is. however, the extremely long time scales for these mergers (∼several billion years) make each observation a single snapshot in the long merger history, and we must infer many of the details necessary for understanding the full merger process. furthermore, current weak lensing analyses lack the precision required to detect a signal from self-interacting dark matter. uncertain weak lensing mass and position estimates also yield large uncertainties in the dynamical reconstruction of the merger scenarios. need: in order to better model the dynamics of merging galaxy cluster systems, and to potentially measure any signal from self-interacting dark matter, we need to obtain more precise measurements on the masses and positions of the dark matter halos involved. gravitational lensing offers a robust method for mapping the mass in these clusters because it directly measures the gravitational field, and does not depend on the dynamical state of the system that has been disturbed in the merger process. of the lensing methods, weak gravitational lensing is the only way that we can probe a wide field and measure the total mass of the cluster. however, the precision of conventional weak lensing techniques is currently limited by shape noise (uncertainty in the shear due to the dispersion in the intrinsic shapes and orientations of unlensed galaxies). a possible avenue forward is to eliminate shape noise as a source of uncertainty in shear measurements via a technique to be described below. this would eliminate the largest source of uncertainty in weak lensing analyses, and enable us to obtain mass and position estimates of dark matter halos with a much higher level of precision. task: in this dissertation we perform statistical clustering, conventional weak lensing analyses, and dynamical reconstruction on the merging galaxy cluster system zwcl 2341.1+0000 in order to test the capabilities of the dynamical modeling on a complex, multiple merger. we use targeted optical spectroscopy to identify cluster member galaxies, which we then use to model the galaxy substructures. we also obtain a dynamical mass estimate using the galaxy velocity dispersions, and perform weak lensing analyses in the forms of aperture densitometry to place an upper bound on the total cluster mass, and multiple nfw profile halo fitting to approximate the masses and positions of the individual dark matter halos present in the merger. the masses, positions, and line of sight velocities of those clusters are then used to constrain the parameters describing the best fit merger scenario, with radio relic positions and polarization used to further tighten those constraints. we also develop a new method for obtaining weak lensing data from individual source galaxies in the form of shear measurements that are independent of shape noise, and direct measurements of the convergence. we accomplish this by simultaneously modeling the pre-lensing velocity and intensity profiles of a lensed, rotating disk galaxy, and the lensing transform required to distort those into the lensed profiles we observe. we test this method with a host of idealized simulations to characterize its capabilities in a best-case scenario and forecast the possible improvements it can bring to the precision of weak lensing analyses on galaxy clusters. (abstract shortened by proquest.). | merging galaxy clusters: a case study of zwcl 2341.1+0000 and the development of a new forward modeled lensing technique |
searches for new resonances whose decays result in top quarks and/or b-quarks cover a wide range of beyond the standard model (bsm) physics. these searches offer great potential as well as significant challenges in reconstructing and identifying the decay products as well as modeling the sm background. this talk presents the latest 13 tev atlas results, covering both resolved- and boosted-jet topologies. | searches for new resonances decaying to heavy-flavour quarks with the atlas detector at √{ s } = 13 tev |
the maunakea spectroscopic explorer (mse) will conduct a suite of observations that probe the particle nature of dark matter by providing critical input in determining the halo mass function, phase-space distribution, and internal density profiles of dark matter halos across all mass scales. n-body and hydrodynamical simulations of cold, warm, fuzzy and self-interacting dark matter suggest that non-trivial dynamics in the dark sector could have left an imprint on structure formation. analyzed within these frameworks, the extensive and unprecedented datasets produced by mse will be used to search for deviations away from cold, collisionless dark matter model. i will highlight a few science cases that will be enabled by mse, such as the impact of low mass substructures on the dynamics of milky way stellar streams in velocity space, the estimates of the density profiles of the dark matter halos of milky way dwarf galaxies using more than an order of magnitude more tracers, determining satellite luminosity function by pinning down the redshifts of dwarf galaxies in the milky way analogs in the low redshift universe, and constraining the halo mass functions via strong lensing measurements at higher redshifts. | astrophysical tests of dark matter with mse |
stellar tidal streams from disrupting globular clusters are among the most dynamically cold systems known. this makes them sensitive to both the global structure of the milky way's gravitational potential and to substructures within it. in particular, a passing dark matter subhalo can cause a gap to form in the stream - even if the subhalo is small enough to contain no stars or gas of its own and therefore be otherwise invisible. detection of these disturbances is a promising way to constrain the subhalo mass function, a key observable dependent on the particle nature of dark matter. i will present recent results forecasting the capabilities of the large synoptic survey telescope and the wide field infrared survey telescope for identifying and characterizing stream gaps and density substructure. we find that both instruments will be able to detect individual gaps in streams resulting from encounters with subhalos as light as 106 solar masses throughout much of the milky way, deep in the regime where alternative dark matter models predict a dramatic suppression of the mass function and beyond the capabilities of other proposed techniques. in addition we find that by using lsst or wfirst the statistical properties of the stream's density structure, caused by its whole history of encounters, can be examined at a level that allows us to infer both the rate of impacts and the existence of any cutoff in the subhalo mass function, two crucial predictions of any dark matter model. | stellar streams and milky way substructure in the era of lsst and wfirst |
constraining the distribution of small-scale structure in our universe will allow us to probe alternatives to the cold dark matter (cdm) paradigm. strong gravitational lensing offers a unique window into small dark matter halos (109 msun) because these halos impart a gravitational lensing signal even if they do not host luminous galaxies. however, the tens of thousands of free parameters in gravitational lensing by a substructure population makes directly evaluating the likelihood intractable. simulation-based inference techniques can return posterior estimates without access to the likelihood, but they require a representative set of data simulations. to that end, we introduce the package paltas which builds on the lensing package lenstronomy to create large datasets of strong lensing images with realistic substructure, observational effects, and galaxy light pulled directly from the hubble space telescope's cosmos field. we use this simulation pipeline to train a neural posterior estimator of the subhalo mass function (shmf) parameters and place constraints on populations of lenses generated using a disjoint set of galaxy sources. we find that by combining our networks with a hierarchical inference framework, we can both reliably infer the shmf across a variety of configurations and scale efficiently to large lens populations. to our knowledge, our work is the first to constrain the shmf on simulations with fully realistic sources and substructure, demonstrating the potential of strong-lens imaging to probe dark matter at small scales. | reconstructing the subhalo mass function from strong gravitational lensing |
tesseract computes concentrations of simulated dark matter halos from volume information for particles generated using voronoi tesselation. this technique is advantageous as it is non-parametric, does not assume spherical symmetry, and allows for the presence of substructure. tesseract accepts data in a number of formats, including gadget-2 (ascl:0003.001), gasoline (ascl:1710.019), and ascii, and computes concentrations using particles volumes, traditional fitting to an nfw profile, and non-parametric techniques that assume spherical symmetry. | tesseract: tessellation-based recovery of amorphous halo concentrations |
within lcdm, dwarf galaxies like the large magellanic cloud are expected to host many dark matter subhalos, several of which should be massive enough to host faint dwarf companions. recent gaia proper motions have confirmed new members of the lmc-system including three classical mstar > 10^5 msun dsph as well as several ultra-faint dwarfs with other candidates awaiting additional kinematic data. we use the fire simulations to study the dark and luminous substructure population of isolated lmc-like hosts and place the gaia results in a cosmological context. by comparing number counts of subhalos in runs with and without baryons, we find that the substructure depletion in these low mass hosts is almost negligible, contrary to what is found on milky-way scales. for our highest resolution runs, 7 of the dark matter subhalos form galaxies with mstar >= 10^4 msun, in good agreement with the 5 observationally inferred pre-infall satellites of the lmc. our simulations suggest different structural properties of lmc-associated ultrafaints than interpreted from observations, raising questions regarding the association to the lmc of a couple orbitally consistent dwarfs. | faint and ultra-faint dwarf satellites of the lmc in the fire |
hayashi (halo-level analysis of the absorption signal in hi) computes the number of absorption features of the 21cm forest using a semianalytic formalism. it includes the enhancement of the signal due to the presence of substructures within minihalos and supports non-standard cosmologies with impact in the large scale structure, such as warm dark matter and primordial black holes. hayashi is written in python3 and uses the cosmological computations package colossus (ascl:1501.016). | hayashi: halo-level analysis of the absorption signal in hi |
the cold dark matter model predicts that the milky way is formed by the accretion and merger of many smaller dwarf galaxies. the process of accretion and merger will leave stellar streams and other substructures in the milky way, which are the debris of the dwarf galaxies. to better understand the formation history of the milky way, it is helpful to detect and study these substructures. in this paper, we give a brief review of the history of studies on the sources of the accretion of the milky way, summarize the different distributions of stellar streams in the spatial coherence, clustered in velocity space and chemical group in the early, middle, and late stages respectively. we introduce the method and history of detection, research of stellar streams, and the substructures from these threephase spaces in detail. then we introduce the studies of two typical substructures. the detailed studies of these two substructures have greatly improved our understanding of the formation history and structure evolution of the milky way. a list of sub-structures such as the stellar streams detected up to now is also provided. finally, we give the prospects of using lamost ii project, csst survey, subaru/pfs, etc., in conjunction with gaia to carry out studies on the accretion and merger history of the milky way. | the research of the milky way's accretion and merger origin |
in the standard cosmological model, the matter content of the universe is dominated by cold dark matter (cdm), collisionless particles that interact with ordinary matter (baryons) only through gravity. gravitationally bound dark-matter halos form hierarchically, with the most massive systems growing through mergers of smaller ones. as structure assembles in this fashion, large dark-matter halos contain smaller-scale substructures in the form of embedded subhalos. i will show that observations of gravitational lensing can be used to map the inner mass distribution of cosmic structures such as galaxy clusters to test these predictions of the cdm paradigm. interestingly, the reconstructed granularity of cluster cores implies an excess of galaxy-galaxy strong lensing (ggsl) probability compared to expectations in the ?cdm cosmological model. the theoretical estimates on the ggsl probability are based on the analysis of hydrodynamical simulations, while the observational measurements are derived from parametric strong lensing reconstructions combining inputs from hst and jwst imaging and muse/vlt spectroscopy. in an attempt to understand this issue, cluster-size halos simulated with different mass and force resolutions and implementing several agn energy feedback schemes have been analyzed. the feedback model has a significant impact on the properties of subhalos and on their ability to produce ggsl effects. however, none of the hydrodynamical simulations studied so far are in agreement with observations. they persistently have difficulty reproducing the stellar mass and the internal structure of cluster galaxies simultaneously. the reported mismatch may indicate an unidentified problem with either prevailing simulation methods or standard cosmology. | tensions on small scales: too many galaxy-galaxy strong lenses in galaxy clusters? |
rr lyrae stars are powerful tracers of galactic structure, substructure, accretion history, and dark matter content. the characteristic photometric variability of rr lyrae stars makes it relatively easy to distinguish them from other objects, and they are excellent standard candles. in this paper, we report on the discovery of distant rr lyrae stars, including some of the most distant stars known in the milky way halo, with galactocentric distances larger than 300 kpc. the data set used in this study consists of precise time-series photometry in the u*, g', i', and z' bands obtained with the megacam imager on the canada-france-hawaii (3.6-m) telescope as part of the next generation virgo cluster survey (ngvs). we use a template light curve fitting method based on empirical sdss stripe 82 rr lyrae data to identify rr lyrae candidates in the ngvs data set. in total, we detect 208 rr lyrae candidates, with heliocentric distances of 20-320 kpc. the halo stellar distribution is consistent with an r-3.9 power-law radial density profile over most of this distance range with no signs of a break. the distribution of these rr lyrae in a period-amplitude plot (bailey diagram) suggests that the mean metallicity of the halo decreases outwards. compared to other recent rr lyrae surveys like pan-starrs-1 (ps1), high-cadence transient survey (hits), and dark energy survey (des), our ngvs study has smaller sky coverage, comparable cadence, and better single-epoch photometric precision. the depth of the ngvs dataset results in our rr lyrae sample being the most complete and robust at large distances, with the best measured pulsation parameters. we have an ongoing spectroscopic follow-up program using the echelle spectrograph and imager (esi) on the keck ii 10-m telescope that targets newly discovered rr lyrae in the distance range 80-150 kpc in the ngvs and other fields. we have spectra in hand for 36 of these stars at the time of the writing of this abstract and are in the process of obtaining more spectra. this sample of stars is an important addition to the handful of secure milky way halo tracers beyond 100 kpc for which there are both reliable distances and kinematics. | distant milky way halo rr lyrae stars in the next generation virgo cluster survey: exploring the outer edge of our galaxy |
we present results for utilizing weak lensing analysis in the identification and classification of the dark matter substructure in galaxy clusters. using a previously developed flexion analysis package (flextool) gravitationally lensed images are analyzed for their flexion signal, the anisotropic inward "bowing" of a lensed object. the measured flexion signal can be decomposed toward a nearest-neighbor lensing galaxy as a means for identifying dark matter structure within the lensing galaxy cluster. a figure-of-merit is developed for classifying the underlying substructure using source object size, measured flexion strength and nearest-neighbor radial distance. the analysis is applied to the abell 2744 and macs j0416.1-2403 galaxy clusters in the hst frontier fields program. this is the first such work that directly identifies individual substructure using weak gravitational flexion. | gravitational flexion measures toward substructure in the frontier fields |
milkyway@home is a 0.5 petaflops volunteer computing platform that is mapping out the density substructure of the sagittarius dwarf tidal stream, the so-called bifurcated portion of the sagittarius stream, and the virgo overdensity, using turnoff stars from the sloan digital sky survey. it is also using the density of stars along tidal streams such as the orphan stream to constrain properties of the dwarf galaxy progenitor of this stream, including the dark matter portion. both of these programs are enabled by a specially-built optimization package that uses differential evolution or particle swarm methods to find the optimal model parameters to fit a set of data. to fit the density of tidal streams, 20 parameters are simultaneously fit to each 2.5-degree-wide stripe of sdss data. five parameters describing the stellar and dark matter profile of the orphan stream progenitor and the time that the dwarf galaxy has been evolved through the galactic potential are used in an n-body simulation that is then fit to observations of the orphan stream. new results from milkyway@home will be presented. this project was supported by nsf grant ast 16-15688, the nasa/ny space grant fellowship, and contributions made by the marvin clan, babette josephs, manit limlamai, and the 2015 crowd funding campaign to support milky way research. | characterizing milky way tidal streams and dark matter with milkyway@home |
the nature of dark matter can be better constrained by observing merging galaxy clusters. however, uncertainty in the viewing angle leads to uncertainty in dynamical quantities such as 3-d velocities, 3-d separations, and time since pericenter. the classic timing argument links these quantities via equations of motion, but neglects effects of nonzero impact parameter (i.e. it assumes velocities are parallel to the separation vector), dynamical friction, substructure, and larger-scale environment. we present a new approach using n-body cosmological simulations that naturally incorporate these effects. by uniformly sampling viewing angles about simulated cluster analogs, we see projected merger parameters in the many possible configurations of a given cluster. we select comparable simulated analogs and evaluate the likelihood of particular merger parameters as a function of viewing angle. we present viewing angle constraints for a sample of observed mergers including the bullet cluster and el gordo, and show that the separation vectors are closer to the plane of the sky than previously reported. | merging galaxy clusters: analysis of simulated analogs |
some strong gravitational lens systems exhibit features that cannot be explained by a smooth mass distribution. a possible explanation for these anomalies is the presence of dark matter substructure of the type predicted by lcdm. much work has been done to use these anomalous lens systems to attempt to detect individual dark matter clumps. the standard approach is to increase the complexity of the model to include a component that can account for a single dark matter clump. some groups go on to use the clumps in their models to constrain the subhalo mass function and abundance, and in doing so assume that the modeled clump corresponds to a physical clump in the lens. our goal is to quantify the reliability of this assumption. we will create realistic mock lenses with substructure, use them to create mock lensed images, and fit those images with single clump models. working with mock lenses means we can quantify how systematic effects may bias the conclusions we draw about the clump's properties. any bias in this step will propagate into the constraints placed on the mass function or abundance of dark matter. we will carry out this analysis for quasar lenses and galaxy lenses. | testing methods to measure dark matter substructure with gravitational lensing |
over the last two decades, strong gravitational lensing has emerged as a promising tool for probing the nature and distribution of dark matter on sub-galactic scales. in addition to the main-lens substructure, dark matter halos along the line-of-sight between the observer and the source contribute significantly to the subtle perturbations of lensed images. these line-of-sight halos, unlike dark matter subhalos, appear stretched tangentially around the einstein radius in the effective convergence maps, imprinting a distinct anisotropic signature. these anisotropies produce a quadrupole moment of the image-plane averaged two-point correlation function of the effective convergence field. we show that current space-based telescopes and future extremely large telescopes can detect this quadrupole signal using fisher forecasting. in addition, we demonstrate how, in a strongly lensed system, this anisotropic signal can statistically separate the contribution of line-of-sight halo perturbations to lensing perturbations from that of main-lens subhalos. finally, we will look at how the anisotropic two-point function changes when warm dark matter and self-interacting dark matter are present. this strategy opens the door to improving constraints on the evolution of dark matter structure on small scales with upcoming large-scale surveys | dark matter physics through small-scale lensing anisotropies |
the splashback radius of a dark matter halo, which refers to the radius that infalling matter and substructures approach the apocenter for the first time, has been detected around galaxy clusters using a multitude of observational methods including galaxy cluster weak lensing measurements. in this poster, we present how the splashback radius affects galaxy cluster masses derived through weak lensing measurements if it is not accounted for. we find that the splashback radius has an increasingly large effect on lower-mass group-sized halos towards 1013.5 m⊙. it is possible for this bias to further propagate into cluster cosmology analysis, resulting in lower ωm values, a bias in the same direction that recent cluster cosmology analyses have discovered, although splashback alone can not sufficiently explain the recently-discovered cosmological bias. the splashback effect with group-sized dark matter halos may become important to consider nevertheless given the increasingly stringent cosmological constraints coming from optical wide-field surveys. | the effect of splashback on weak lensing mass estimates of galaxy clusters and groups |
we present preliminary results to test the accuracy of association methods applied to galaxy surveys, in order to constraint their limitations and develop possible alternatives to improve them. we focused in the friends-of-friends (fof) method, carrying on the analysis on the dark matter cosmological simulation mdpl2, from the multidark project. results point to a large fraction of contaminants from the application of the fof method, particularly for massive haloes in high density environments. thresholds in the association parameters and the subsequent use of tests for substructures can mitigate the occurance of fake positives. | siblings, friends and acquaintances: testing the galaxy association methods |
this thesis explores wave dark matter and its intriguing phenomenological implications, namely the presence of wave-like signatures on astrophysical length scales. specifically, we assume the dark matter is composed of light bosons with masses below 10 ev, allowing, due to the high phase space occupation number, for a classical wave description. we develop a formalism to describe the wave dark matter's response to gravity, accounting for the statistical properties of the field. our first subject of investigation is the behavior of the wave dark matter in the solar system. here we study the phenomenon of gravitational focusing, where a massive astrophysical object deforms the local distribution of dark matter, leading to a local overdensity. we calculate the response of observationally-motivated dark matter substructures to the sun's potential and find unique signatures in the local overdensity and dark matter spectrum that can be relevant for direct detection ex- periments. the second topic of discussion is the behavior of wave dark matter in a small dark matter halo. in particular, we investigate the wave dark matter response to the adiabatic growth of a black hole in the center of the halo. this phenomenon leads to a compression of the surrounding dark matter halo, resulting in a steeper density profile. we find significant wave features in the density profile of the compressed halo's inner region, where the semiclassical approximation breaks down. as an application, we investigate the gravitational waves produced by the inspiral of a compact solar-mass object with a central intermediate-mass black hole within the compressed wave dark matter halo. due to the enhanced mass density, the compressed halo exerts dynamical friction on the orbiting object which is stronger than in the uncompressed case, leading to a characteristic dephasing of the gravitational waves. this quantity being sensitive to the underlying dark matter model, we dis- cuss concrete scenarios where the wave dark matter halo can be reconstructed from gravitational wave observations. | gravitational signatures of wave dark matter |
merger trees track the evolution of halos across multiple snapshots. they assign for halos of a particular snapshot, the set of halos from previous snapshots they possibly originated from. in this work, association rule analysis a well known technique from data mining has been used to build halo merger trees. association rule analysis tries to find associations between different halos(in same as well as in snapshots) using the particle ids of the particles which the halos are made of. associations are expressed in the form of association rules. merger trees are one of the several useful results one can obtain from the output of association rule analysis. other results including halo substructure and halo splitting can also be extracted. each type of output to be extracted from the association rule analysis output correspond to a pattern in association rules. merger trees were formed and tested using the above technique. dark matter simulations were run using gadget-2 for 128^3 particles. halos were extracted from the simulation snapshots using amiga halo finder. halo accretion history was plotted and compared against those formed using ahf merger tree builder. | application of association rule analysis to study the evolution of halos in cosmological n-body simulations |
the wealth of ongoing and upcoming observations of the milky way (mw) promise an era of "near-field cosmology" that can test the cold dark matter (cdm) paradigm. one of the most exciting and powerful probes of dm is using the mw's stellar streams as "gravitational antennae:" close passages of small dm substructures dynamically perturb cold stellar streams, allowing us to test the diverging predictions of different dm models for the low-mass end of the (sub)halo mass function. recent measurements of mw stellar streams offer exciting hints of evidence for dm subhalo perturbations, but a critical challenge in modeling and interpreting such subhalo-stream interactions remains: the lack of sufficiently realistic theoretical tools to make robust observational predictions in realistic mw-like environments. we propose to model the dynamics of dm subhalo interactions with stellar streams from disrupted globular clusters and satellite galaxies in realistic detail, by implementing new numerical methods to inject stellar streams into existing cosmological zoom-in hydrodynamic simulations of mw-like galaxies, and creating synthetic observations of these simulated perturbed streams. these simulations will allow us to analyze, for the first time, the gravitational and hydrodynamical effects of a live mw-like galaxy on the survivability of nearby dm subhalos, as well as baryonic sources of perturbations on stellar streams (such as giant molecular clouds) that can mimic those ascribed to dm substructure. using these new tools, we will make detailed predictions for the observable dynamical effects of dm subhalo interactions on stellar streams. furthermore, we will resimulate several of our mw systems using alternative (warm or self-interacting) dm models to generate falsifiable observational tests of the nature of dm via the analysis of stellar streams. the proposed work will result in the first comprehensive, end-to-end study that connects cosmological predictions from hydrodynamic simulations to interpretations of, and predictions for, observable perturbations from dm subhalos on stellar streams. new numerical tools developed for this proposal will allow us to predict the degree to which observations of stream perturbations uniquely constrain the properties of the dm subhalo perturbers, and how observable stream properties differ in alternative dm models. we will make our synthetic observations of streams, subhalo catalogs, and full simulation snapshot data publicly available, in user-friendly databases, providing much-needed theoretical tools to the community for exploring and testing subhalo-stream dynamics at this critical time for mw observations. | predicting observable signatures for dynamical interactions between dark-matter substructure and stellar streams in the milky way |
the lambda cold dark matter model for structure formation has been very successful at reproducing observations of large scale structures. however, challenges emerge at sub-galactic scales, e.g. the number of dwarfs around the milky way show an order of magnitude difference with simulations (the 'missing satellites problem'). there are several theories to explain this apparent discrepancy but further observations of local volume galaxies and their substructure is required to constrain these models by better sampling halo to halo scatter. here we report on a survey of the m101 group from archival data and a novel dwarf detection algorithm. this survey has discovered 26 new dwarf candidates in the m101 system, extending the dwarf luminosity function by two magnitudes, to m=-7.5. these dwarf candidates also show a distinct spatial asymmetry suggestive of an infalling dwarf group. | a new dwarf detection algorithm applied to m101 |
weakly interacting massive dark matter (dm) particles are expected to self-annihilate or decay, generating high-energy photons in these processes. this establishes the possibility for indirect detection of dm by \gamma-ray telescopes. for probing the secondary products of dm, accurate knowledge about the dm density distribution in potential astrophysical targets is crucial. in this contribution, the prospects for the detection of subhalos in the galactic dm halo with present and future imaging atmospheric cherenkov telescopes (iact) are investigated. the source count distribution and angular power spectra for \gamma-rays originating from annihilating dm in subhalos are calculated from n-body simulation results. to study the systematic uncertainties coming from the modeling of the dm density distribution, parameters describing the \gamma-ray yield from subhalos are varied in 16 benchmark models. we conclude that galactic subhalos of annihilating dm are probably too faint to be a promising target for iact observations, even with the prospective cherenkov telescope array (cta). | search for galactic dark matter substructures with cherenkov telescopes |
the cosmological model based on cold dark matter (cdm) and dark energy has been hugely successful in describing the observed evolution and large scale structure of our universe. however, at small scales (in the smallest galaxies and at the centers of larger galaxies), a number of observations seem to conflict with the predictions of cdm galaxy formation theory, leading to a recent interest in alternative dark matter models. i will show that a proper consideration of the physics of ordinary matter (gas and stars) can significantly alter the dark matter structure and substructure of galaxies, alleviating the small scale problems. i will examine what future observations can help to put constraints on the nature of dark matter. | re-examining astrophysical constraints on the dark matter model |
arising from inhomogeneities during inflation, primordial black holes (pbh) have long been proposed as a dark matter candidate. there has been a renewed interest in pbhs after the ligo detections of binary black hole mergers, with claims that these were primordial in nature. gravitational wave observations hold promise for future scrutiny of these theories: pbh binaries produce gravitational waves detectable by lisa. we evolve pbh clusters using a direct n-body simulation, including post-newtonian physics and gravitational wave recoil, to capture the evolution of dense pbh substructures. we present preliminary results on the predicted gravitational wave strain and evolution of the pbh mass function. | gravitational wave radiation from primordial black hole clusters |
in most lux data analyses, the collaboration have mostly relied on the lux data processing framework's output known as the ''reduced quantities'' (e.g. event energy, position etc.). however some info embedded in the unprocessed multichannel photomultiplier tube (pmt) time traces were lost in the processing. to extract these info and improve current existing analysis, a technique uses convolutional neural networks (cnn) in the discrimination between a single s2 vertex vs. a double s2 vertex was developed. after training the cnn with all 122 channels of pmt waveforms, it can correctly identify double s2s that are partially merged together and look like a single vertex but with substructure. such merged vertex signal would arise from two simultaneous particle interactions that occur within a few mm. implementing this new identification technique not only reduces the hand scanning effort required, but also improves our analyses in (1) eliminating backgrounds from conventional source, and (2) identifying rare event signals. the preliminary results of applying this technique to different physics searches such as xenon isotopes double decay half life estimate will be presented. department of energy. | machine learning applied to background events identification in lux dark matter experiment |
ultralight bosons with de broglie wavelength 1 kpc have been proposed as a model of dark matter (dm) which can potentially evade some problems on small scales (core-cusp, substructure). here we investigate a dwarf dm halo evolution in a tidal field of a host halo taking into account the dm quantum tunneling. this process reduces the dwarf halo mass by tunneling it into the host halo. we demonstrate that, for a halo of a particular mass, the process is very sensitive to the dwarf's radial position. we argue that the observed distribution of satellites around milky way and andromeda can strongly constrain and even rule out the ultralight dm model. supported by the doe epscor grant de-sc0019474, the doe grant de-sc0016368, and by kitp via the nsf grant phy-1748958. | dwarf halos and ultralight dark matter model |
in this thesis, i study the expected direct and indirect detection signals of dark matter. more precisely, i study three aspects of dark matter; i use hydrodynamic simulations to extract properties of weakly interacting dark matter that are relevant for both direct and indirect detection signals, and construct viable dark matter models with interesting experimental signatures. first, i analyze the full scale illustris simulation, and find that galactic indirect detection signals are expected to be largely symmetric, while extragalactic signals are not, due to recent mergers and the presence of substructure. second, through the study of the high resolution milky way simulation eris, i find that metal-poor halo stars can be used as tracers for the dark matter velocity distribution. i use the sloan digital sky survey to obtain the first empirical velocity distribution of dark matter, which weakens the expected direct detection limits by up to an order of magnitude at masses $\lesssim 10$ gev. finally, i expand the weakly interacting dark matter paradigm by proposing a new dark matter model called boosted dark matter. this novel scenario contains a relativistic component with interesting hybrid direct and indirect detection signatures at neutrino experiments. i propose two search strategies for boosted dark matter, at cherenkov-based experiments and future liquid-argon neutrino detectors. | boosting (in)direct detection of dark matter |
this thesis presents a search for dark matter production in association with a higgs boson decaying to a pair of bottom quarks, using data from 20.3 fb-1 of proton-proton collisions at a center-of-mass energy of 8 tev collected by the atlas detector at the lhc. the dark matter particles are assumed to be weakly interacting massive particles, and can be produced in pairs at collider experiments. events with large missing transverse energy are selected when produced in association with high momentum jets, of which at least two are identified as jets containing b-quarks consistent with those from a higgs boson decay. to maintain good detector acceptance and selection efficiency of the signal across a wide kinematic range, two methods of higgs boson reconstruction are used. the higgs boson is reconstructed either as a pair of small-radius jets both containing b-quarks, called the "resolved'' analysis, or as a single large-radius jet with substructure consistent with a high momentum b b system, called the "boosted'' analysis. the resolved analysis is the focus of this thesis. the observed data are found to be consistent with the expected standard model backgrounds. the result from the resolved analysis is interpreted using a simplified model with a z' gauge boson decaying into different higgs bosons predicted in a two-higgs-doublet model, of which the heavy pseudoscalar higgs decays into a pair of dark matter particles. exclusion limits are set in regions of parameter space for this model. model-independent upper limits are also placed on the visible cross-sections for events with a higgs boson decaying into bb and large missing transverse momentum with thresholds ranging from 150 gev to 400 gev. | search for dark matter produced in association with a higgs boson decaying to two bottom quarks at atlas |
with the completion of 7 tev and 8 tev data taking at the large hadron collider (lhc), the physics community witnessed one of the great triumphs of modern physics: the completion of the standard model (sm) as an effective theory. the final missing particle, the higgs boson, was observed and its mass was measured. however, many theoretical questions remain unanswered. what is the source of electroweak symmetry breaking? what is the nature of dark matter? how does gravity fit into the picture? with no definitive hints of new physics at the lhc, we must consider the possibility that our search strategies need to be expanded. conventional lhc searches focus on theoretically motivated scenarios, such as the minimal supersymmetric standard models and little higgs theories. however, it is possible that new physics may be entirely different from what we might expect. in this thesis, we examine a variety of scenarios that lead to new physics undercover at the lhc. first we look at potential new physics hiding in quantum chromo-dynamics backgrounds, which may be uncovered using jet substructure techniques in a data-driven way. then we turn to new long-lived particles hiding in higgs decay, which may lead to displaced vertices. such a signal can be unearthed through a data-driven analysis. then we turn to new physics with ``semi-visible jets'', which lead to missing momentum aligned with jet momentum. these events are vetoed in traditional searches and we demonstrate ways to uncover these signals. lastly, we explore performance of future colliders in two case studies: stops and higgs portal searches. we show that a 100 tev collider will lead to significant improvements over 14 tev lhc runs. indeed, new physics may lie undercover at the lhc and future colliders, waiting to be discovered. | new physics undercover at the lhc |
we request hst/acs imaging to follow up the 10 most prominent newly discovered substructures in the halos of ngc 253 and ngc 5128 (both at d=3.8 mpc, and the dominant galaxies of their respective groups, sculptor and centaurus a). the substructures were found via our ongoing ground-based (magellan/megacam) survey out to 150 kpc for each galaxy, and include stunning disrupting satellites, streams and shells. current cold dark matter based simulations provide testable predictions for the physical properties (e.g ages and metallicities) of in-situ/accreted components, but the lack of a robust observational census of outer halos for a wide range of galaxies beyond the local group prevents robust conclusions. our proposed observations of the halo substructures in ngc 253 and ngc 5128 will confirm their distance, group membership, and allow for an accurate derivation of their luminosities, metallicities, and star formation histories: for the first time, substructures around milky way-sized halos beyond the local group will be characterized in great detail. the substructures' properties will be compared to those of the smooth halo, as derived from our wide-field imaging as well as from extant hst halo observations, and to similar substructures in the halos of the milky way and m31. this program will constitute a critical observational test to cosmological predictions of the hierarchical assembly process. | resolved halo substructures beyond the local group: the assembly histories of ngc 253 and ngc 5128 |
both the milky way (mw) and its massive neighbor, m31, host systems of satellite dwarf galaxies that are relics of their assembly history. nasa observatories, such as hst, wfirst, and jwst, and survey missions, such as gaia and lsst, have/will be used to identify new substructure about the mw and m31 and measure the kinematics of local group members with unprecedented accuracy. the ultimate goal for these data is to learn about the assembly and structure of the dark matter halos of the mw and m31. to this end, this substructure (e.g., satellite galaxies) is typically considered as isolated, point mass-tracers of their host potentials. this assumption, however, breaks down if the total mass of the satellite is a significant fraction of the host mass, which is the case for the lmc and m33. such massive satellites will modify and contribute to the dark matter distribution of the host, changing the shape of the halo potential in a non-symmetrical, time-evolving manner. as such, the lmc and m33 have the potential to perturb the kinematics of all tracers of the halo potential of both m31 and the mw, such as globular clusters, halo stars, stellar streams, and smaller satellites orbiting in the halo. to date, this effect has not been accounted for in models of the local group. the goal of this nasa atp program is to use high-resolution, n-body simulations to build time-evolving models of the dark matter distribution about the mw and m31, owing to the evolution of their massive satellites. analytic realizations of the resulting potentials will be released to the community. in addition, this program will place these new models of the local group and the evolution of massive satellites in a cosmological context. astrophysicists do not currently have an appropriate theoretical framework to study the assembly history of our local group. the proposed detailed theoretical study of how massive dwarfs influence the dark matter halo of their hosts is thus both timely and critical to all efforts to understand the structure and evolution of the dark matter distribution of the mw, m31 and analogous systems. | the impact of massive satellites on the dark matter halos of the milky way and m31 |
a critical challenge to the cold dark matter (cdm) paradigm is that there are fewer satellites observed around the milky way than found in simulations of dark matter substructure. we show that there is a match between the observed satellite counts corrected by the detection efficiency of the sloan digital sky survey (for luminosities l ≥ 340 l⊙) and the number of luminous satellites predicted by cdm, assuming an empirical relation between stellar mass and halo mass. the "missing satellites problem'', cast in terms of number counts, is thus solved. we also show that warm dark matter models with a thermal relic mass smaller than 4 kev are in tension with satellite counts, putting pressure on the sterile neutrino interpretation of recent x-ray observations. importantly, the total number of milky way satellites depends sensitively on the spatial distribution of satellites, possibly leading to a "too many satellites" problem. measurements of completely dark halos below 108 m⊙, achievable with substructure lensing and stellar stream perturbations, are the next frontier for tests of cdm. | there is no missing satellites problem |
the high altitude water cherenkov observatory (hawc) is a high energy (500 gev to 100 tev) gamma ray detector located in central mexico. hawc operates via the water cherenkov technique and has both a wide ( 2 sr) field of view and near continuous duty cycle, making it ideal for unbiased sky surveys and analysis of highly extended sources. we made use of hawc's survey abilities to search for dark matter signals originating from the galactic halo. by taking an unbiased sample of the sky, we set characteristic upper limits on dark matter annihilation as a function of declination. these limits are then used to calculate the hawc sensitivity to dark matter emission at various points in the sky and calculate the hawc sensitivity to emission from dark matter substructure assuming a particular model. with this sensitivity model, we find the optimal regions for various dark matter spatial profiles of the galactic main halo for a dark matter search with hawc. we perform a search for dark matter gamma rays in this region and set upper limits on dark matter annihilation in the galactic halo. | a search for dark matter gamma rays in the galactic halo with hawc |
baryonic feedback at high redshifts has been proposed to explain the inference of dark matter (dm) cores in low-surface brightness and dwarf spheroidal galaxies. however, in the currently favoured cosmological model, structure grows hierarchically and cdm predicts a myriad of small substructures orbiting dwarf galaxies, some luminous (which habe been observationally identified in recent years), some dark. if such dark subhalos get close enough to the centre of cored dwarfs, they could potentially lead to cusp regrowth.in this talk, i will present the evolution of the dm profiles of dwarf galaxies driven by the accretion of dm substructures through controlled n-body experiments. the initial conditions assume that supernova feedback erases the primordial dm cusps at high redshift of halos with final masses $10^{9}-10^{10} \rm{m_{\odot}}$ by z=0. the orbits and masses of the infalling substructures are borrowed from the {\it aquarius} cosmological simulations. i will show that some halos that undergo 1:3 down to 1:30 mergers are susceptible to reform a dm cusp by $z\approx 0$ and how this depends on the internal structure of the infalling substructures. i will show that within cdm a non-negligible level of scatter in the mass profiles of dwarfs is to be expected given their stochastic mass accretion histories and their diverse (observed) star formation histories and that this effect could possibly explain the existence of dense dwarfs like draco, ursa minor or tucana. i will argue how this process is unique to cdm and may be falsifiable. if time allows, i will show preliminary results from suites of cosmological n-body simulations designed to address the statistics of this effect. | dark subhalo accretion onto dwarf galaxies in cdm |
the cold dark matter paradigm predicts that a milky way-like galaxy should be surrounded by thousands of low-mass dark matter halos, many of which are expected to be devoid of stars and gas. the presence of these invisible halos can be inferred using strong gravitational lensing, since dark matter substructure can produce visible perturbations in gravitationally lensed images. the mass we infer for these perturbing halos depends on the halo's assumed redshift, which is often unknown. however, we have shown there is a characteristic radius at which the halo's enclosed mass can be measured robustly. by modeling simulated gravitational lensed data, we address the questions: what range of halo masses and redshifts can we expect to be detectable for a given telescope resolution? for detected perturbations, how well can we reproduce the robust mass? this characteristic scaling may help to reduce bias when detecting and modeling dark matter halos in real world gravitational lenses. | detecting and constraining low mass dark matter halos by their perturbations of gravitationally lensed images |
lcdm predicts that the dark matter subhalo mass function is nearly self-similar. we therefore expect massive galaxies as well as smaller, dwarf galaxies to have a substantial amount of dark matter subhalos surrounding them. however, we expect the occurrence and nature of debris structures around dwarfs to be different than around larger galaxies, as their current accretion rate should be lower, they should have fewer subhalos and their baryon to dark matter fraction should be lower. as a consequence, we anticipate that observed differences in debris structures will offer new insights into hierarchical structure formation on small scales as well as how the baryons occupy dark matter structures around dwarfs, which is largely unexplored. future surveys such as wfirst and lsst will enable us to probe the stellar halos of dwarf galaxies and potentially detect substructure surrounding dwarfs. in this talk, i will discuss our expectations for the importance of stellar halos and the frequency of streams around lmc-type dwarf galaxies by estimating the merger rate and halo occupation from cosmological simulations. additionally, i will compare the fraction of stars in these galaxies anticipated to be formed in-situ in the main halo progenitor vs accreted from other subhalos. thus, i will assess whether we will find streams around dwarf galaxies with the next generation telescopes and discuss what we might learn if we find these structures. | the frequency of stellar streams around dwarf galaxies |
one outstanding question in cosmology is, what are the smallest galaxies that can form? the answer to this question can tell us much about galaxy formation, and even of the properties of dark matter itself. a candidate for the smallest galaxies that can form are the ultrafaint galaxies. the star formation of ultrafaints appears to have been shut off during the epoch of reionization, when radiation from the first stars ionized all the free hydrogen in the universe. this would imply ultrafaints should exist everywhere in the universe. however, we can only observe ultrafaints as satellites of the milky way, due to their low brightness. this will change with the next generation of telescopes such as the large synoptic survey telescope (lsst). the focus of this work is to predict the number of ultrafaints that should be seen with future surveys. to that end, we use the elvis suite, which contains 14 dark matter only simulations of local group like systems containing a milky way and andromeda-like galaxy and the substructure out to around 1 mpc of the barycenter. we mock observe the simulations in order to mimic current surveys such as the sloan digital sky survey (sdss), and the dark energy survey (des), and use the population of galaxies found by those surveys to project the population of dwarf galaxies out beyond the virial radius of either galaxy. this number will depend sensitively on the formation mechanism of ultrafaint dwarfs, and comparisons of future surveys to this work could help rule out certain formation scenarios. | the smallest galaxies in the universe: investigating the origins of ultra-faint galaxies |
axions, which can solve the strong cp problem, and axion-like particles (alps), which arise naturally in many models of high-scale physics, provide theoretically compelling dark matter candidates. axions and alps which couple to photons have been shown to produce observable radio emission through their conversion to photons in the magnetospheres of neutron stars, providing a means of indirect detection. in this work, we analyze 1 hour of radio data collected by the effelsberg 100-m radio telescope to place novel constraints on μev axion dark matter. we also briefly discuss implications of dark matter substructure for this search strategy. | constraints on axion dark matter from searches for radio signals from neutron stars |
i will present a dynamical analysis of abell 267 (z~0.23) using over 1000 galaxy redshifts from new spectra observed with magellan/m2fs combined with publicly available redshifts from the hectospec cluster survey. for each galaxy, we measure redshift as well as mean age, metallicity, alpha enrichment, and internal velocity dispersion of the stellar population. we applied a new method to simultaneously obtain bayesian estimates for the internal kinematics and substructure of the cluster. for the main cluster population, we implement a jeans analysis in order to fit the cluster's dark matter halo, which is embedded within a mixture model that explicitly accounts for member and contamination galaxies as well as any subpopulations. we find that with this comprehensive model, we are able to disentangle effects of subpopulations and cluster rotation, revealing a relatively clear view of abell 267's dynamical status and gravitational potential. | galaxy cluster mass estimates in the presence of substructure |
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