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We investigate the implications of the intergalactic opacity for the evolution of the cosmic ultraviolet luminosity density and its sources. Our main constraint follows from our measurement of the Lyman-α forest opacity at redshifts EQUATION from a sample of 86 high-resolution quasar spectra. In addition, we impose the requirements that intergalactic HI must be reionized by EQUATION and HeII by EQUATION, and consider estimates of the hardness of the ionizing background from HI to HeII column density ratio measurements. The derived hydrogen photoionization rate is remarkably flat over the forest redshift range covered. Because the quasar luminosity function is strongly peaked near EQUATION, the lack of redshift evolution indicates that star-forming galaxies likely dominate the photoionization rate at EQUATION, and possibly at all redshifts probed. Combined with direct measurements of the galaxy UV luminosity function, this requires only a small (emissivity weighted) fraction EQUATION of galactic hydrogen ionizing photons to escape their source for galaxies to solely account for the entire ionizing background. Under the assumption that the galactic UV emissivity traces the star formation rate (which is the case if the escape fraction and dust obscuration are constant with redshift), current state-of-the-art observational estimates of the star formation rate density, which peak similarly to quasars at EQUATION, appear to underestimate the total photoionization rate at EQUATION by a factor EQUATION, are in tension with the most recent determinations of the UV luminosity function, and fail to reionize the Universe by EQUATION if extrapolated to arbitrarily high redshift. A star formation history peaking earlier, as in the theoretical model of Hernquist & Springel, fits the forest photoionization rate well, reionizes the Universe in time, and is in better agreement with the rate of EQUATION gamma-ray bursts observed by Swift. Quasars suffice to doubly ionize helium by EQUATION, provided that most of the HeII ionizing photons they produce escape into the intergalactic medium, and likely contribute a non-negligible and perhaps dominant fraction of the hydrogen ionizing background at their EQUATION peak. |
Finally, we use estimates of the spectrum of the softness of the UV background from measurements of the HI to HeII column density ratio obtained by combining HI and HeII forest spectra. |
Although the thermal history of the IGM may only have a modest effect on the gas density PDF, the parameters EQUATION and β must be known in order to unambiguously solve for the evolution of the background hydrogen photoionization rate, Γ. In this paper, we do not attempt to infer EQUATION and β directly from the data; rather, we use existing measurements, which we combine with our theoretical understanding of the thermal physics of the IGM in order to assess the robustness of our conclusions. We begin by reviewing this physics before examining the present observational constraints. |
Note that the systematic uncertainties in the temperature of the IGM at the mean density EQUATION and on the slope of the temperature-density relation β, especially if they conspire to affect the inferred Γ in the same direction, could go some way in resulting in a declining photoionization rate toward high redshifts EQUATION and therefore bringing it in better agreement with observationally derived star formation rates (see 18). However, the requirement that the Universe be reionized by EQUATION also argues forcefully against a steep decline of star formation beyond EQUATION ( 20). The latter argument, depending only on the magnitude of Γ at EQUATION through the normalization of the ionizing emissivity, is largely insensitive to the uncertainties in the thermal history at the redshifts probed by our forest opacity measurement. |
We finally note that even for identical choices of (EQUATION, β, γ), our estimate of the mean free path would exceed that of ARTICLE by about 30% owing to their assumption of an Einstein-de Sitter cosmology, in contrast to the WMAP cosmology we adopt. |
Although equation 28 appears to assume isotropic quasar emissivity, it is more generally valid. For instance, if quasars emit with constant intensity in some directions and are completely obscured toward the others, then the overestimate of the total photon output of a quasar based on its observed magnitude and the assumption of isotropy will be exactly canceled by the fraction of quasars missed in the luminosity function. To generalize further, note that in a context in which quasars shine with different intensities in different directions, the observed quasar luminosity function can be interpreted as quantifying the number of (randomly oriented) lines of sights of given intensity from quasars. For the purpose of calculating the cosmological emissivity, one can imagine distributing these sight lines among isotropically emitting sources and recover equation 28. |
To compare the photoionization rate measured from the forest to what can be accounted for by the observed quasars and estimated star formation history, we proceed as follows. We begin by fixing the quasar contribution, with the working assumption that it is better constrained than that of stars. For each star formation history we consider, we then solve for the constant EQUATION in equation (34) that minimizes the EQUATION between our measured Γ and the sum of the quasar and stellar contributions. We repeat the exercise for different normalizations of the quasar contribution to investigate how our conclusions would be affected if the escape fraction of quasars were less than unity, or if there were an error in the normalization of the mean free path or in the assumed spectral indexes. |
Many early calculations of the evolution of the global ionized fraction during reionization assumed clumping factors derived from averaging over the entire gas content of high-resolution simulations, which were as large as EQUATION at the redshifts EQUATION relevant for hydrogen reionization. Such clumping factors are however unrealistically high, as they include gas in collapsed halos which are associated with the galaxies that produce the ionizing photons. As the escape fraction EQUATION already accounts for the effects of clumpy gas within these sources, the latter should not be included in calculating the clumping factor used to calculate the recombinations in the diffuse IGM. |
The conclusions of the present section are in general agreement with the similar analysis of ARTICLE, who found that the ionizing comoving emissivity cannot decline from EQUATION up to EQUATION by more than a factor 1.5 if reionization is to be complete at EQUATION. Our results also support the picture of a ``photon-starved'' reionization similarly advocated by ARTICLE based the EQUATION forest photoionization rate. |
Second, the vast majority of the absorbers for which the column density ratio η has been measured are optically thin to both HI and HeII ionizing photons, whereas the systems that are optically thick to HeII ionizing photons are likely to contribute significantly to limiting their mean free path. As the expected column density ratio η varies non-monotonically with increasing column density ratio EQUATION near the optically thick transition for reasonable background spectra, it is unclear in which direction our optically thin calculation for the mean free path ratio may be biased. |
The arguments of this section have thus far fundamentally relied on the shape of the redshift evolution of the quasar emissivity and of the total photoionization rate. This is the reason we used analytic fits (which can be straightforwardly extrapolated to higher redshifts) to the star formation history, a quantity which at high redshifts is derived from the observed UV luminosity functions. As the UV luminosity functions themselves are the relevant quantities that involve the least unnecessary assumptions, it is of interest to compare them directly with the UV emissivity implied by our forest measurement by equation (47). |
In Figure 53, we show both the SFR density derived from galaxy UV luminosity functions and from the UV emissivity derived from our forest opacity measurement, assuming the best-fit escape fraction of 40. We find no compelling evidence for a decline in the comoving SFR density over the redshift range probed by our measurement, either from it or from the directly measured UV luminosity function, in contrast to the best fit of ARTICLE. Although the star formation rate derived from the forest is subject to a significant uncertainty on the mean free path of ionizing photons, our assumption for the redshift evolution of the latter is conservative and the discrepancy would be exacerbated if Lyman limit systems instead evolved more similarly to optically thin systems ( 32). |
Inspection of Figure 53 suggests that the present data are instead roughly consistent with a constant EQUATION at EQUATION. Since our analysis assumes a dust correction consistent with these authors at high redshifts, but is based on more recent data, it thus seems that (in combination with the arguments given in 18 and 44) the SFR density peak suggested by their fit is an artifact of the scarce high-redshift data in their compilation, which may be affected by cosmic variance and is not uniformly complete. For example, one of the EQUATION points that drive the ARTICLE fit is the estimate of ARTICLE, which is only complete to EQUATION and is based on an extremely small HUDF 11 arcmin² exposure. We instead consider the analysis of ARTICLE, which includes the HUDF data as a subset and yields a higher SFR density, and consistently integrate the luminosity functions down to zero luminosity. |
An intriguing possibility is that the very high redshift Universe harbors a large number of extremely faint galaxies that are missed by even the deepest surveys to date ARTICLE. As we have integrated the measured luminosity functions down to zero luminosity, this would require a significant steepening of the faint-end slope of the UV luminosity function below the present observational limits. Faint galaxies may in fact already have been detected in abundance in the deep EQUATION searches of ARTICLE and ARTICLE, although the authenticity of the candidates remain challenging to confirm and their large number poses challenges to theoretical expectations. The recent detection of very faint Lyman emitters at EQUATION by ARTICLE that would have been missed in existing LBG surveys also reminds us that the very dim Universe may contain surprises. |
1. The intergalactic hydrogen photoionization rate is remarkably constant over the redshift range EQUATION, with a value EQUATION sEQUATION, subject to remaining systematic uncertainties in normalization. 2. Because the quasar luminosity function is strongly peaked near EQUATION, the lack of redshift evolution implies that star-forming galaxies are likely to dominate the photoionizing background at EQUATION, and possibly at all redshifts probed. 3. Although our arguments are robust to normalization systematics, fiducial assumptions regarding the mean free path of ionizing photons, the spectral energy distribution of quasars, and an escape fraction of ionizing photons from quasars of unity imply that quasars alone overproduce the total ionizing background near their peak. This puzzle suggests that these assumptions warrant more scrutiny and highlights the uncertainties in calculating the ionizing background by integrating luminosity functions. 4. Only a small escape fraction of ionizing photons EQUATION is needed for galaxies to solely account for the entire UV luminosity density implied by the forest, with the scaling with uncertain parameters given in equation 57. This small fraction might be reconciled with higher measurements from the direct detection of Lyman-continuum photons from high-redshift galaxies if it increases with galaxy luminosity. 5. The state-of-the-art observational fit to the cosmic star formation history by ARTICLE (peaking near EQUATION similarly to quasar activity) appears to underestimate the total photoionization rate by almost an order of magnitude at EQUATION if the escape fraction and dust obscuration are constant with redshift for EQUATION, and is also in tension with the most recent high-redshift determinations of the galaxy UV luminosity function. 6. Normalizing the ionizing emissivity predicted by the best-fit star formation history of ARTICLE to our forest measurement at EQUATION, their star formation history fails to reionize the Universe by EQUATION. 7. A star formation history peaking at a higher redshift EQUATION, like the theoretical model of ARTICLE, fits the EQUATION forest well, reionizes the Universe in time, and is in better agreement with the rate of GRBs observed by Swift at EQUATION. 8. Quasars alone suffice to doubly ionize helium by EQUATION provided that most of the HeII ionizing photons they produce escape into the IGM. 9. The ratio the HI to the HeII column densities in the post-HeII reionization epoch EQUATION suggests, with a significant uncertainty arising from the large fluctuations observed, that quasars contribute a non-negligible fraction (EQUATION%) and perhaps dominate the hydrogen ionizing background at their EQUATION peak. |
Although the present uncertainties in the redshift evolution of UV dust corrections, the escape fraction of ionizing photons from galaxies, and possible evolution of the stellar initial mass function however prohibit definitive conclusions about the cosmic SFR to be reached at this time, we have illustrated how the IGM represents a powerful integral probe of the cosmic luminosity density free of the completeness issues that affect the direct detection of its sources. In this sense, its study is very complementary to that of luminosity functions. |
Better constraints on the thermal history of the IGM and on the abundance of the Lyman-limit systems which determine the mean free path of ionizing photons will be key in reducing the systematic uncertainties in the arguments presented here. In addition, improved determinations of the spectral indices of quasars and star-forming galaxies, especially shortward of the Lyman limit, will be useful in refining our calculations. In the same vein, an updated calculation of the full spectrum of the ionizing background, like that of ARTICLE, is clearly warranted in the light of our improved empirical knowledge of the physical state of the IGM and of the luminosity functions of both quasars and galaxies (Faucher-Gigu\`ere et al., in prep.). |
In Figure 5 we plot the mean values of a and e over the span of the simulations for the different cases, with and without the gas disk. While the inner planets show no clear separation in distribution, there is a consistent difference in eccentricity for the outer planets. The arrows connect the properties of the outer planet for each case, with and without remnant disks. In each instance, the simulation with a disk ends up at a lower eccentricity. The effect of the disk on the inclination evolution of the systems is much less clear, with no consistent trend appearing in our simulations. |
We have used widely spaced in time Hubble Space Telescope images to determine tangential velocities of features associated with outflows from young stars. These observations were supplemented by groundbased telescope spectroscopy and from the resultant radial velocities, space velocities were determined for many outflows. Numerous new moving features were found and grouped into known and newly assigned Herbig Haro objects. It was found that stellar outflow is highly discontinuous, as frequently is the case, with long term gaps of a few hundred years and that these outflow periods are marked by staccato bursts over periods of about ten years. Although this has been observed in other regions, the Orion Nebula Cluster presents the richest display of this property. Most of the large scale Herbig Haro objects in the brightest part of the Orion Nebula appear to originate from a small region northeast of the strong Orion-S radio and infrared sources. With the possible exception of HH 203, we are not able to identify specific stellar sources, but do identify candidate sources for several other bright Herbig Haro objects. We find that there are optical features in the BN-KL region that can be related to the known large scale outflow that originates there. We find additional evidence for this outflow originating 500–1000 years ago. |
The long operational life of the Hubble Space Telescope (HST) has presented the opportunity to obtain and compare emission-line images of the central (Huygens) region of the Orion Nebula over a sufficiently long time-base to determine highly accurate motions in the plane of the sky. These motions enable us to study the outflow that accompanies early phases of star formation and the interaction of this out-flowing plasma with the ambient nebular material. An introduction to the Orion Nebula and its associated cluster is presented in several recent review articles ARTICLE. |
In this paper we present the results of the determination of new tangential and radial velocities and their interpretation. We will show that a number of new stellar outflows (Herbig Haro objects, HH) have been found, that some previously designated HH objects are not what was originally argued, and that these motions provide us with important new information about star formation in the visually obscured BN-KL and Orion-S centers. |
Our observational material consists of both new spectroscopic and imaging data. The spectroscopic observations were made at Observatorio Astron\'omico Nacional en San Pedro M\'artir operated by the Universidad Nacional Aut\'onoma de M\'exico using the MEZCAL echelle spectrograph ARTICLE. The new imaging observations were made with the HST's WFPC2 and ACS cameras as part of program GO 10921. The data were processed with the IRAF package of software. |
The Huygens region of the Orion Nebula has been the subject of many spectroscopic studies. Most of these did not have velocity resolutions to determine with sufficient accuracy the radial velocities of features moving about the speed of sound (about 10 ) and the studies with this necessary spectral resolution usually treated only certain sections of the nebula. More complete high resolution mapping of the Huygens region have been made recently [3, 6, 29] in the strongest lines. Those multi-slit-setting maps were made with primarily north-south oriented slits, so that the spatial resolution was excellent along the slit length, but poorer in right ascension because the spacing was usually wider than the characteristic ``seeing". In this study, we draw upon some of those earlier spectra, but our primary target was an extremely detailed east-west region centered on the Orion-S area and new observations were necessary. |
These new observations were made over the period 11–14 January 2007. The instrument configuration and performance was the same as in our earlier study of the Ring Nebula ARTICLE. The spectrograph was employed in a single spectral order mode by isolating the order containing the 6563 and [N II] 6548 and 6583 lines with a narrow-band interference filter. The 70 μm wide entrance slit projected 0.9 onto the plane of the sky. We processed the well exposed sample of 187. The 24 μm pixels of the Site 1024x1024 CCD detector were double-sampled along the slit (0.62 per pixel). The spectrum was read out in single pixels along the dispersion direction, giving Full Width at Half Maximum (FWHM) of the Thorium-Argon comparison lamp emission lines of about 7.0. The atmospheric seeing varied from one to one and one half arcseconds during the exposures. A series of east-west oriented slit settings were made using offsets from the bright star 159-350=JW 499 (in the position based system introduced in ARTICLE or the catalog of ARTICLE) and at intervals of about 1 at locations shown in Figure 1. We will use the position based system of designation throughout this paper, adding decimal places when an accuracy of greater than about 1 is needed. Exposures of 600 s were made with characteristic peak signals of 30,000 counts in. Transparency variations were corrected by normalizing each slit's signal against a similar region of a flux calibrated ARTICLE image of this object ARTICLE made with the HST's WFPC2, but gaussian blurred to resemble ground-based image quality. Figure 1 shows contour plots of the resulting spectra. |
Our imaging program was planned to allow a homogenous set of data to be obtained, which was intended to allow the best possible comparison of earlier and later images. In planning the observations it was recognized that the high spatial resolution of the ACS-WFC (0.05 / pixel in the drizzled images) was better than that of the WFPC2 (0.0996 / pixel), but the latter instrument allows a longer time base since it was installed earlier. The planning became more complex as the ACS-WFC failed and WFPC2 observations were substituted where possible. |
In this paper we report on the analysis of new images in four areas of the Orion Nebula, supplemented by similar investigation of pairs of earlier observations. The four areas were selected to coincide as much as possible with earlier images of what were thought to be the most interesting and relevant objects. The characteristics of the new observations are summarized in Table 3. The total exposure times presented were obtained in triple exposures, which allowed removal of cosmic ray induced spots on the images. On-the-fly routinely processed data were the starting point for our study. |
We drew on a large body of earlier observations made with the WFPC2 and the ACS-WFC. Their characteristics are presented in Table 4. This listing includes not only the images matched with the new observations, but also images that were reprocessed and analyzed as part of this program. In addition to these long time-base ACS-WFC images, we also employed archived F658N ACS-WFC images obtained through program GO 10246 with Massimo Robberto as principal investigator. |
The first step in comparison of the first and second epoch images was to align the two. In each case we took the first epoch images as the standard and shifted the second epoch images. Since the individual CCD detectors of both cameras can change with respect to their neighbors, alignment and comparison was only made using reference stars falling into a single CCD in both the early and later images. This is the same technique used in our earlier investigations ARTICLE using data with a shorter time interval. The accuracy of the alignment was typically about 0.2 pixels, which corresponds to about the 3 internal dispersion of velocities of the Orion Nebula Cluster stars [2, 32]. Because the first and second epoch fields of view were not exactly on the same part of the sky, the region useful for comparisons is more irregular and smaller than the second epoch images. |
After alignment the signal of the two images was normalized and a search was made for moving objects by taking a ratio of the first and second epoch images. In the case of identical images, the ratio image would be a smooth value of unity and in the case where objects are moving there would be a pattern with a dark leading edge and a bright trailing edge. In most cases motion of individual features was measured using the least squares method ARTICLE employed previously, or when the objects were large or the regions complex, simply measuring their positions directly. The measurement uncertainties ranged from 0.1 to 0.4 pixels, depending on the nature of the object being measured. The uncertainty in the direction of the motion is less well defined, but can be 30 for very small motions and only a few degrees for the large motions. The results of the measurement of discrete objects are presented in Table 5 (Appendix B) and Table 6 (Appendix C). However, there were many large-scale moving features not measured in either fashion, but the reality of the motion in the ratio images is unquestionable. |
Figure 3 and Figure 4 show ratio images for our Visit 1 and Visit 2 images for the WFPC2 F656N and F658N filters, supplemented by images near HH 203 and HH 204 from a comparison of archived GO 5193 and GO 8121 images which have been smoothed by median filtering. The difference in time between the first and second epoch images was 12.76 years for Visit 1, 12.49 years for the Visit 2 F631 and F673N images, 13.71 years for the Visit 2 F502N, F547M, F656N, and F658N images, and 6.25 years for the HH 203/204 images. At our adopted distance of 440 pc (c.f. Appendix A), these intervals correspond to WFPC2 velocity scales of 16.3, 16.6, 15.2, and 33.2 per WFPC2 pixel respectively. Ratio images in F502N were created and used for finding rapidly moving objects but are not shown here as all moving features in this filter are also seen in the F656N image, whereas many of the F656N moving features are not seen in the F502N image. Figure 5 shows the same tangential velocity image superimposed on a WFPC2 color image. |
The detailed coding of the superimposed data for Figure 3, Figure 4, and Figure 5 are described here, rather than in the figure captions. The markings on the outer borders show the Declination south of EQUATION and the Right Ascension east of 5EQUATION 35EQUATION. Filled circles indicate the positions of stars mentioned in the discussion and those within the dashed outline are found in the near infrared catalog of ARTICLE. Arrows indicate the direction and magnitude of the tangential velocities, with the arrow length corresponding to the motion in 300 years. The colors of the arrows are added only to indicate local groupings, the same color being used multiple times for different groups. The dashed elongated rectangular outline indicates the region of radial velocity results shown in Figure 2. Open red circles indicate the position of knots ARTICLE and red lines features seen in Subaru images ARTICLE. The long irregular blue lines delineate collimated high velocity flow as indicated by He I 10830 emission [9, 36]. The strong dark red/blue contoured lines indicate CO outflow ARTICLE and the pastel colored pink/light-blue contoured lines indicate SiO outflow ARTICLE. Open squares indicate the positions of H₂O maser sources ARTICLE. Point sources within the dashed outline are coded by the shortest wavelength of their detection, with filled white squares indicating the positions of radio only visible sources [37, 39, 40], red squares the positions of sources seen only in the mid infrared [41, 42], and filled orange circles positions of stars in the near infrared catalog of ARTICLE. A few motions outside of our field of view have been added with the coding OD=ARTICLE, DOH=ARTICLE, and G-DH=ARTICLE. The region most likely to contain the sources of the high velocity optical outflows is shown as a dark dashed ellipse. |
Figure 6 shows the Visit 5 region of overlap with the GO 9460 field near HH 502. In this case the time interval was 4.29 years and the velocity scale 24.3 per ACS-WFC pixel. It has had the faint fine features enhanced by division by a third order polynomial fit to the background. The nomenclature is that of ARTICLE. |
This rich body of new data allows us to draw conclusions about many aspects of gas-flow and star formation in the Orion Nebula Cluster (ONC) and its secondary star formation regions in the BN-KL and Orion-S regions. In this discussion we will first consider outflows associated with the Orion-S region (both as individual systems and as a possible global unit). We then treat what the new data tell us about the BN-KL region. The multiple outflows to the south of the Huygens region and centered on HH 502 are discussed third. The discussion concludes with consideration of individual proplyds and outflow systems. Throughout this paper we will assume that the velocity of the host Orion Molecular Cloud (OMC) is=25.8 ARTICLE and that of the ONC is 25.6 ARTICLE. Velocities relative to the OMC will be designated as. We will continue to assume that the distance is 440 pc, so that one-tenth parsec subtends 46.9. Position Angles (PA) will be expressed in the ordinary way, counter-clockwise from north and θ, the angle of flow with respect to the plane of the sky, is positive in the direction of the observer. We draw heavily on published slit-spectroscopy maps of higher ionization ARTICLE and lower ionization lines ARTICLE (henceforth DOH04 and GD07), and where we do not have new tangential velocities, on ARTICLE, henceforth DOH, or ARTICLE, henceforth OD03. |
There is by now a rich literature about large-scale (greater than 0.1 pc) outflows probably originating from the Orion-S region, the idea going back to the symmetric forms of HH 202 and HH 203+HH 204 being pointed in opposite directions across Orion-S, then developing as more outflows (HH 269, HH 529, HH 528, HH 625) were found. ARTICLE, henceforth HO07, have most recently summarized the history and characteristics of these outflows, including their possible connection to much larger apparent shocks outside of the Huygens region, so that it is not necessary to discuss each object anew, beyond summarizing how the new observations have altered our understanding. Our new observations do give valuable new information about the exact region of the sources of these outflows. |
Once again we find the well known result that all of the large-scale outflows are blue-shifted with respect to the OMC. When their current motions are projected backwards, these projected lines cross in the Orion-S regions, suggesting that they have a common origin in one or more objects located there. The results for these objects are summarized in Table 7. The region that probably contains the sources of the high velocity optical outflows is shown as an ellipse in Figure 3, Figure 4, and Figure 5. This region is a more refined estimate of the location and properties of the OOS (Optical Outflow Source) identified in ARTICLE. |
HH 202. HH 202 is at the apex of a large parabolic high ionization feature, as shown in Figure 5. The variety of ionization stages indicates that although it forms as a collimated high velocity outflow it also contains neutral material. This can be due to the flow impacting neutral material or that the flow has compressed the ambient ionized gas to the degree that it traps the ionization front. If the latter, this would require that the material lies within the main cavity of the nebula rather than in the foreground veil. The white arrows in this region in Figure 5 indicate tangential motions in the HH 202-South brightest part of the object and the red arrows the HH 202-North part. The yellow arrow indicates flow in a shock feature ahead of HH 202-North (designated as HH 202-NW) and the blue and orange arrows two shocks even further ahead (designated as HH 202-WNW). The collimated flow probably driving these shocks has been measured by DOH04 to have radial velocities of about EQUATION to EQUATION and the details of this flow are best shown in the He I 10830 line ARTICLE, which is seen in good contrast against the background emission from the nebula because the line is usually quite optically thick except where the emitting materially is Doppler shifted off the core of the line. |
HH 203. HH 203 is a well defined bow shock with low ionization features at its tip. It is apparently driven by a high velocity, high ionization jet that emerges into the zone ionized by or the nearer (in the plane of the sky) (which dominates the ionization of some of the proplyds in this area) about half the distance from the point of origin. The jet radial velocity is EQUATION. |
HH 204. HH 204 is nearly at the same PA as HH 203, but shows marked differences. Its apex is a flocculent structure that includes low ionization features even though the body of the enclosed parabolic form has extended [O III] emission, indicating that it is forming in ionized gas. No driving jet is seen. It should be noted that HO07 found a broader shock front extending beyond even HH 204 and at a PA midway between HH 204 and the terminus of HH 528, as if there had been three events of collimated flow in about this direction. |
HH 269. HH 269 is composed of a series of well defined, wide-ionization range, shocks oriented almost due west ARTICLE. In this study, only features near 116-345 were measured (the so-called HH 269-East shock) but both in tangential and radial velocity. A 5 narrow but wiggly low ionization feature was also measured at its western terminus and is designated here as HH 269-Ram. |
HH 528. HH 528 shows a broad band of irregular, low ionization, structures headed southeast from the Orion-S region. It is composed of two regions, the base or ``jet'' (as designated in HO07) and the southeast-most bow shock (again as designated in HO07). The term ``jet'' only loosely applies as this feature is much broader than any collimated flow producing the other HH objects in this study and one sees individual bow shocks within the object. Falling back on the original discovery work notation for HH 528 ARTICLE where it was noted that the entire object is shaped like a mushroom, we will here designate the ``jet'' as the ``base'' and the end feature as the ``cap''. HH 528 may in fact represent two outflows, the ``base'' object defined by broad irregular structure, oriented towards PA=155 and containing features moving towards PA=178 and a second outflow represented by the ``cap'', oriented very approximately towards 147 and containing features moving towards 159. In both cases the objects may be passing through the low-ionization portion of the nebula, rather than beomg within the main body of photoionized gas. A mediating argument against division of HH 528 is that the velocities and spatial orientation of the two components are very similar. |
There is a peculiar complex of features around 5:35:17.6 EQUATION:24:54 (all coordinates in this paper will be in epoch 2000). Parts of this complex seem related to the HH 528 flow, such as the 177-454 shock, which is measured in [S II] to be moving at 37 towards PA=199. However, there are other features that seem to be moving at velocities of order 130 towards PA= 100. Most puzzling is a feature just to the southeast of 177-454, where a naive interpretation of the [N II] ratio map would imply an oppositely directed motion of 130 towards PA= 280. However, careful inspection of the individual images in multiple lines suggests that this feature is also moving towards PA= 100. The reason for the discrepant behavior in [N II] is that in this filter the feature seems to be a moving dark feature rather than a moving bright feature, which reverses the sense of motion in the ratio image. In addition, the morphology of the feature seems to be evolving, becoming darker in the second epoch. It is much larger and more diffuse than the shock features associated with HH 528 and at this point remains unexplained. |
HH 529. HH 529 is composed of a series of shocks oriented towards the east and moving in that direction. Here we break it down into designated shocks HH 529-III, HH 529-II, and HH 529-I proceeding from east to west (as shown in Figure 2–Figure 5). There is a series of small, rapidly moving features which may represent the driving, nearly collimated outflow that we designate as HH 529-Ram. There may be an HH 529-0 shock which is not obvious on images, but reveals itself through high radial velocity features a few arcseconds north and south of the HH 529-Ram group (cf. Figure 2). The HH 529-Ram features begin at a Right Ascension of 5:35:14.91 and end at 5:35:15.6 while the putative HH 529-0 shock is at 5:35:15.5, i.e. near the west end of the Ram features, and its velocity EQUATION is comparable to the Rams' EQUATION, which also argues for an association. Bow shocks HH 529-III, HH 529-II, and HH 529-I are high ionization only, indicating that they fall within the zone of ionization by, a conclusion supported by the detailed spectroscopic analysis of HH 529-III+HH 529-II by ARTICLE. However, the HH 529-Ram feature is seen in both high and low ionization lines, indicating that this outflow is moving from a low into a high ionization region. |
The brightest infrared and radio sources lie to the southwest of the putative source or sources producing the multiple large-scale HH objects associated with Orion-S. Recent radio observations reveal two well defined molecular outflows in this southwest part of Orion-S. CO observations ARTICLE detail a bipolar outflow with the blueshifted component towards PA=305 and apparently arising from the infrared star 136-400, which has associated H₂O masers ARTICLE. A second study in SiO reveals a bipolar molecular outflow that apparently arises from the infrared star 136-355, with its blueshifted component towards PA=284 ARTICLE. These features are shown in Figures 3–5. There are several spots of SiO emission in the strong radio sources 137-408 (CS 3) and 134-411 (FIR 4), but no obvious directed outflow. 136-400 and 136-355 have no optical counterparts and they must lie behind a significant amount of obscuring material, so one would expect to find optical components to the molecular outflows associated with only the blueshifted components. |
The SiO blueshifted component coming from 136-355 intersects with the HH 269 series of shocks, which have an orientation of 273 ARTICLE, an orientation strengthened by the presence of long filaments ARTICLE. The feature for which we have measured the tangential velocity lies upon a projection of the SiO blueshifted outflow, but the direction of motion of these features (PA=275) more nearly agrees with the overall orientation of HH 269 and a line projected back to the HH 269-Ram feature. |
There is a well defined series of optical and infrared features associated with the CO blueshifted component. One sees features extending towards 136-400 and there are a series of knots ARTICLE extending out to 204 (0.44 pc). There is a peculiar, low ionization, optical feature that has been designated as HH 625 and we measured four features in it with an average motion of=34 towards PA=296. We have examined and [N II] spectra of this region ARTICLE and determined that EQUATION. This means that this optical feature (which must be formed in a region of lower obscuration than applies closer to 136-400) has a velocity vector of 48 inclined 41 towards the observer with respect to the plane of the sky. The CO observations trace the blueshifted component to EQUATION (EQUATION), which means that the optical feature probably represents mass-loaded material moving more slowly than the original outflow. However, the small size of the knots indicates that the flow that drives them has remained very narrow and collimated. |
In Figure 7 we show the central portion of the Huygens region that includes Orion-S. This incorporates all of the area where the axes of symmetry of the flows (ordered counter-clockwise) of HH 202, HH 529, HH 203, HH 204, HH 528, HH 269, and HH 625 intersect, this general area having been previously identified as the most likely source for these outflows ARTICLE. This figure is similar to Figure 5, except the color balance is slightly different, we have added labels identifying the individual sources, and tangential velocities are shown projected for only 100 years. |
Although we have derived higher accuracy tangential velocities, these have not narrowed down the identification of the sources of the large outflows except that various outflows seem to originate in the eastern part of this small field. If the features we call in this paper the HH 269-Ram and HH 529-Ram are from the same source, its position is narrowed down to a Right Ascension interval of only 15. However, to link these two flows as a single bipolar flow demands that one of the sides has been deflected towards the observer by about 116. Although a mechanism that provides deflection as a beam passes through a density gradient has been proposed ARTICLE, it seems a stretch to use this as the process that allows linking HH 269 and HH 529 into a single bipolar flow. The infrared source 146-351 and far infrared source 144-351 are the top candidates for the sources of these outflows. Although the other outflows must also arise in this same region, linking them all into a single source requires invoking the same large amount of deflection multiple times, which is unlikely. Multiple infrared and radio sources exist in this region and if they individually produce bipolar flows, then the observational selection effect of seeing only blueshifted outflow from a source lying behind the main ionization front would apply and explain the dominance of blueshifts. If the extinction that causes the sources to be visible only in the infrared was local to the source, then one would expect to see both redshifted and blueshifted extended flow. |
A recent infrared polarization study ARTICLE may provide additional useful information. ARTICLE argue that imbedded stars with bipolar outflows create dumbbell shaped cavities along the inner parts of the outflow and that starlight scattered from the surface of these cavities would have a characteristic polarization pattern. They have studied the infrared polarization in this region and find polarization signatures around 136-355, 145-356, and 144-351, making them prime candidates for outflows. Possible outflow from 145-356 or the star 145-355 only 0.9 north of it is discussed in 3.8. The direction of the putative flow from these two stars is indistinguishably the same as the high velocity flow feeding HH 203. The east side of the polarization pattern associated with 144-351 is generally oriented to be like a flow in the direction of the HH 529-Ram features, thus strengthening the argument that this is the source for the HH 529 flow. |
In the southwest portion of Figure 7 one finds the sources of the molecular outflows, 136-400 producing the bipolar CO flow, whose northwest blueshifted portion creates the optical object HH 625 plus the series of H₂ knots ahead of it, and 136-355, producing the SiO outflow, portions of which may be in the features we are calling the western end of HH 269. The polarization pattern for 136-355 looks perpendicular to the SiO outflow direction, so its interpretation is not clear. Neither of the brighter radio sources (137-408, CS 3; 134-411, FIR 4) in the corner of Figure 7 have clearly associated optical outflows, although the shocks constituting HH 530 and shown in Figures 3–5, appear to move away from the region of 137-408 and 134-411. |
Study of the BN-KL region is primarily the subject of infrared and radio investigations since most of outflowing material and all of the luminous central sources are invisible at optical wavelength. However, optical studies have aided in narrowing down the origin [7, 46]. Study of the motion of the three most massive sources in BN-KL shows that they are expanding away from a common point [11, 47] which we label as ``DYN-CTR'' in our figures. The timescale for expansion of the optically visible shocks is 1000 years ARTICLE, although this may be shorter if the material has been decelerated ARTICLE. ARTICLE find a dynamic timescale of about 500 years. The cause of this explosive decay and the physics relating it to the molecular outflow are still unresolved, but it is reasonable to link the outflow to whatever occurred there. An alternative view is that of ARTICLE, who argue that SMA1 is the source of the high velocity flow, with a dynamic timescale of about 1000 years, and that source I is the source of the low velocity outflow. |
End of preview.
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