John A. R. Caldwell

The Hubble Ultra Deep Field

This paper presents the Hubble Ultra Deep Field (HUDF), a 1 million s exposure of an 11 arcmin2 region in the southern sky with the Advanced Camera for Surveys on the Hubble Space Telescope using Directors Discretionary Time. The exposure time was divided among four filters, F435W (B435), F606W (V606), F775W (i 775), and F850LP (z850), to give approximately uniform limiting magnitudes mAB ~ 29 for point sources. The image contains at least 10,000 objects, presented here as a catalog, the vast majority of which are galaxies. Visual inspection of the images shows few if any galaxies at redshifts greater than ~4 that resemble present-day spiral or elliptical galaxies. The image reinforces the conclusion from the original Hubble Deep Field that galaxies evolved strongly during the first few billion years in the infancy of the universe. Using the Lyman break dropout method to derive samples of galaxies at redshifts between 4 and 7, it is possible to study the apparent evolution of the galaxy luminosity function and number density. Examination of the catalog for dropout sources yields 504 B435 dropouts, 204 V 606 dropouts, and 54 i775 dropouts. The i775 dropouts are most likely galaxies at redshifts between 6 and 7. Using these samples, which are at different redshifts but derived from the same data, we find no evidence for a change in the characteristic luminosity of galaxies but some evidence for a decrease in their number densities between redshifts of 4 and 7. Assessing the factors needed to derive the luminosity function from the data suggests that there is considerable uncertainty in parameters from samples discovered with different instruments and derived using independent assumptions about the source populations. This assessment calls into question some of the strong conclusions of recently published work on distant galaxies. The ultraviolet luminosity density of these samples is dominated by galaxies fainter than the characteristic luminosity, and the HUDF reveals considerably more luminosity than shallower surveys. The apparent ultraviolet luminosity density of galaxies appears to decrease from redshifts of a few to redshifts greater than 6, although this decrease may be the result of faint-end incompleteness in the most distant samples. The highest redshift samples show that star formation was already vigorous at the earliest epochs at which galaxies have been observed, less than 1 billion years after the big bang.

The size evolution of galaxies since z~3: combining SDSS, GEMS, and FIRES

We present the evolution of the luminosity-size and stellar mass-size relations of luminous ( L-V greater than or similar to 3.4 x 10(10) h(70)(-2) L-circle dot) and massive ( M-* greater than or similar to 3 x 10(10) h(70)(-2) M-circle dot) galaxies in the last similar to 11 Gyr. We use very deep near-infrared images of the Hubble Deep Field-South and the MS 1054-03 field in the J(s), H, and K-s bands from FIRES to retrieve the sizes in the optical rest frame for galaxies with z > 1. We combine our results with those from GEMS at 0.2 < z < 1 and SDSS at z similar to 0.1 to achieve a comprehensive picture of the optical rest-frame size evolution from z = 0 to 3. Galaxies are differentiated according to their light concentration using the Sersic index n. For less concentrated objects, the galaxies at a given luminosity were typically similar to 3 +/- 0.5 ( +/- 2 sigma) times smaller at z similar to 2: 5 than those we see today. The stellar mass-size relation has evolved less: the mean size at a given stellar mass was similar to 2 +/- 0.5 times smaller at z similar to 2.5, evolving proportionally to ( 1 + z) - 0.40 +/- 0.06. Simple scaling relations between dark matter halos and baryons in a hierarchical cosmogony predict a stronger ( although consistent within the error bars) than observed evolution of the stellar mass-size relation. The observed luminosity-size evolution out to z similar to 2.5 matches well recent infall model predictions for Milky Way-type objects. For low-n galaxies, the evolution of the stellar mass-size relation would follow naturally if the individual galaxies grow inside out. For highly concentrated objects, the situation is as follows: at a given luminosity, these galaxies were similar to 2.7 +/- 1.1 times smaller at z similar to 2.5 ( or, put differently, were typically similar to 2.2 +/- 0.7 mag brighter at a given size than they are today), and at a given stellar mass the size has evolved proportionally to ( 1 + z)(-0.45 +/- 0.10).

GEMS: The Surface Brightness and Surface Mass Density Evolution of Disk Galaxies

We combine HST imaging from the GEMS (Galaxy Evolution from Morphologies and SEDs) survey with photometric redshifts from COMBO-17 to explore the evolution of disk-dominated galaxies since z 1.1. The sample is composed of all GEMS galaxies with Sersic indices n < 2.5, derived from fits to the galaxy images. We account fully for selection effects through careful analysis of image simulations; we are limited by the depth of the redshift and HST data to the study of galaxies with MV -20, or equivalently, log 10. We find strong evolution in the magnitude-size scaling relation for galaxies with MV -20, corresponding to a brightening of ~1 mag arcsec-2 in rest-frame V band by z ~ 1. Yet disks at a given absolute magnitude are bluer and have lower stellar mass-to-light ratios at z ~ 1 than at the present day. As a result, our findings indicate weak or no evolution in the relation between stellar mass and effective disk size for galaxies with log 10 over the same time interval. This is strongly inconsistent with the most naive theoretical expectation, in which disk size scales in proportion to the halo virial radius, which would predict that disks are a factor of 2 denser at fixed mass at z ~ 1. The lack of evolution in the stellar mass-size relation is consistent with an inside-out growth of galaxy disks on average (galaxies increasing in size as they grow more massive), although we cannot rule out more complex evolutionary scenarios.

GEMS: Galaxy Fitting Catalogs and Testing Parametric Galaxy Fitting Codes: GALFIT and GIM2D

In the context of measuring the structures of intermediate-redshift galaxies with HST ACS surveys, we tune, test, and compare two widely used fitting codes (GALFIT and GIM2D) for fitting single-component Sersic models to both simulated and real galaxy data. Our study focuses on the GEMS survey with the sensitivity of typical HST survey data, and we include our final catalog of fit results for all 41,495 objects detected in GEMS. Using simulations, we find that fitting accuracy depends sensitively on galaxy profile shape. Exponential disks are well fit and have small measurement errors, whereas fits to de Vaucouleurs profiles show larger uncertainties owing to the large amount of light at large radii. Both codes provide reliable fits with little systematic error for galaxies with effective surface brightnesses brighter than that of the sky; the formal uncertainties returned by these codes significantly underestimate the true uncertainties (as estimated using the simulations). We find that GIM2D suffers significant systematic errors for spheroids with close companions owing to the difficulty of effectively masking out neighboring galaxy light; there appears to be no work-around to this important systematic in GIM2Ds current implementation. While this crowding error affects only a small fraction of galaxies in GEMS, it must be accounted for in the analysis of deeper cosmological images or of more crowded fields with GIM2D. In contrast, GALFIT results are robust to the presence of neighbors because it can simultaneously fit the profiles of multiple companions as well as the galaxy of interest. We find GALFITs robustness to nearby companions and factor of 20 faster runtime speed are important advantages over GIM2D for analyzing large HST ACS data sets.

The STAGES view of red spirals and dusty red galaxies: mass-dependent quenching of star formation in cluster infall

We investigate the properties of optically passive spirals and dusty red galaxies in the A901/2 cluster complex at redshift ∼0.17 using rest-frame near-ultraviolet–optical spectral energy distributions, 24-μm infrared data and Hubble Space Telescope morphologies from the STAGES data set. The cluster sample is based on COMBO-17 redshifts with an rms precision of σcz ≈ 2000 km s −1 . We find that ‘dusty red galaxies’ and ‘optically passive spirals’ in A901/2 are largely the same phenomenon, and that they form stars at a substantial rate, which is only four times lower than that in blue spirals at fixed mass. This star formation is more obscured than in blue galaxies and its optical signatures are weak. They appear predominantly in the stellar mass range of log M∗/M� = [10, 11] where they constitute over half of the star-forming galaxies in the cluster; they are thus a vital ingredient for understanding the overall picture of star formation quenching in clusters. We find that the mean specific star formation rate (SFR) of star-forming galaxies in the cluster is clearly lower than in the field, in contrast to the specific SFR properties of blue galaxies alone, which appear similar in cluster and field. Such a rich red spiral population is best explained if quenching is a slow process and morphological transformation is delayed even more. At log M∗/M� < 10, such galaxies are rare, suggesting that their quenching is fast and accompanied by morphological change. We note that edge-on

An Explanation for the Observed Weak Size Evolution of Disk Galaxies

Surveys of distant galaxies with the Hubble Space Telescope and from the ground have shown that there is only mild evolution in the relationship between radial size and stellar mass for galactic disks from z ~ 1 to the present day. Using a sample of nearby disk-dominated galaxies from the Sloan Digital Sky Survey (SDSS) and high-redshift data from the GEMS (Galaxy Evolution from Morphology and SEDs) survey, we investigate whether this result is consistent with theoretical expectations within the hierarchical paradigm of structure formation. The relationship between virial radius and mass for dark matter halos in the ΛCDM model evolves by about a factor of 2 over this interval. However, N-body simulations have shown that halos of a given mass have less centrally concentrated mass profiles at high redshift. When we compute the expected disk size-stellar mass distribution, accounting for this evolution in the internal structure of dark matter halos and the adiabatic contraction of the dark matter by the self-gravity of the collapsing baryons, we find that the predicted evolution in the mean size at fixed stellar mass since z ~ 1 is about 15%-20%, in good agreement with the observational constraints from GEMS. At redshift z ~ 2, the model predicts that disks at fixed stellar mass were on average only 60% as large as they are today. Similarly, we predict that the rotation velocity at a given stellar mass (essentially the zero point of the Tully-Fisher relation) is only about 10% larger at z ~ 1 (20% at z ~ 2) than at the present day.

THE EVOLUTION OF EARLY-TYPE RED GALAXIES WITH THE GEMS SURVEY: LUMINOSITY-SIZE AND STELLAR MASS-SIZE RELATIONS SINCE z = 1

We combine imaging from the Hubble Space Telescope Advanced Camera for Surveys, as part of the GEMS (Galaxy Evolution from Morphologies and SEDs) survey, with redshifts and rest-frame quantities from COMBO-17 to study the evolution of morphologically early-type galaxies with red colors since z = 1. From 05 ? 05 imaging, we draw a large sample of 728 galaxies with centrally concentrated radial profiles (i.e., n ? 2.5 from S?rsic fits) and rest-frame (U - V) colors on the red sequence. We explore how the correlations of rest-frame V-band luminosity and of stellar mass with intrinsic half-light size change over the last half of cosmic time. By appropriate comparison with the well-defined local relations from the Sloan Digital Sky Survey, we find that the luminosity-size and stellar mass-size relations evolve in a manner that is consistent with the passive aging of ancient stellar populations. By itself, this result is consistent with a completely passive evolution of the red early-type galaxy population. If instead, as demonstrated by a number of recent surveys, the early-type galaxy population builds up in mass by roughly a factor of 2 since z ~ 1, our results imply that new additions to the early-type galaxy population follow similar luminosity-size and stellar mass-size correlations, compared to the older subset of early-type galaxies. Adding early-type galaxies to the red sequence through the fading of previously prominent disks appears to be consistent with the data. Through comparison with models, the role of dissipationless merging is limited to <1 major merger on average since z = 1 for the most massive galaxies. Predictions from models of gas-rich mergers are not yet mature enough to allow a detailed comparison to our observations. We find tentative evidence that the amount of luminosity evolution depends on galaxy stellar mass, such that the least massive galaxies show stronger luminosity evolution compared to more massive early types. This could reflect a different origin of low-mass early-type galaxies and/or younger stellar populations; the present data are insufficient to discriminate between these possibilities.

Bar Evolution over the Last 8 Billion Years: A Constant Fraction of Strong Bars in the GEMS Survey

Original article can be found at: --http://www.journals.uchicago.edu/--Copyright The American Astronomical Society

Less than 10 percent of star formation in z=0.6 massive galaxies is triggered by major interactions

Both observations and simulations show that major tidal interactions or mergers between gas-rich galaxies can lead to intense bursts of star formation. Yet, the average enhancement in star formation rate (SFR) in major mergers and the contribution of such events to the cosmic SFR are not well estimated. Here we use photometric redshifts, stellar masses, and UV SFRs from COMBO-17, 24 mu m SFRs from Spitzer, and morphologies from two deep Hubble Space Telescope (HST) cosmological survey fields (ECDFS/GEMS and A901/STAGES) to study the enhancement in SFR as a function of projected galaxy separation. We apply two-point projected correlation function techniques, which we augment with morphologically selected very close pairs (separation = 10(10) M(circle dot)) star-forming galaxies at 0.4 < z < 0.8, we find that the SFRs of galaxies undergoing a major interaction (mass ratios <= 1:4 and separations <= 40 kpc) are only 1.80 +/- 0.30 times higher than the SFRs of non-interacting galaxies when averaged over all interactions and all stages of the interaction, in good agreement with other observational works. Our results also agree with hydrodynamical simulations of galaxy interactions, which produce some mergers with large bursts of star formation on similar to 100 Myr timescales, but only a modest SFR enhancement when averaged over the entire merger timescale. We demonstrate that these results imply that only less than or similar to 10% of star formation at 0.4 <= z <= 0.8 is triggered directly by major mergers and interactions; these events are not important factors in the build-up of stellar mass since z = 1.

Cosmological weak lensing with the HST GEMS survey

We present our cosmic shear analysis of {GEMS}, one of the largest wide-field surveys ever undertaken by the Hubble Space Telescope. Imaged with the Advanced Camera for Surveys ({ACS)}, {GEMS} spans 795 arcmin2 in the Chandra Deep Field South. We detect weak lensing by large-scale structure in high-resolution {F606W} {GEMS} data from {\textasciitilde}60 resolved galaxies per square arcminute. We measure the two-point shear correlation function, the top-hat shear variance and the shear power spectrum, performing an {E/B} mode decomposition for each statistic. We show that we are not limited by systematic errors and use our results to place joint constraints on the matter density parameter ??m and the amplitude of the matter power spectrum ??8. We find ??8(??m/0.3)0.65= 0.68 +/- 0.13 where the 1?? error includes both our uncertainty on the median redshift of the survey and sampling variance. Removing image and point spread function ({PSF)} distortions are crucial to all weak lensing analyses. We therefore include a thorough discussion on the degree of {ACS} {PSF} distortion and anisotropy which we characterize directly from {GEMS} data. Consecutively imaged over 20 d, {GEMS} data also allow us to investigate {PSF} instability over time. We find that, even in the relatively short {GEMS} observing period, the {ACS} {PSF} ellipticity varies at the level of a few per cent which we account for with a semi-time-dependent {PSF} model. Our correction for the temporal and spatial variability of the {PSF} is shown to be successful through a series of diagnostic tests.