3D-HST+CANDELS: The Evolution of the Galaxy Size-Mass Distribution since $z=3$

Spectroscopic + photometric redshifts, stellar mass estimates, and rest-frame colors from the 3D-HST survey are combined with structural parameter measurements from CANDELS imaging to determine the galaxy size-mass distribution over the redshift range 0<z<3. Separating early- and late-type galaxies on the basis of star-formation activity, we confirm that early-type galaxies are on average smaller than late-type galaxies at all redshifts, and find a significantly different rate of average size evolution at fixed galaxy mass, with fast evolution for the early-type population, R_eff ~ (1+z)^-1.48, and moderate evolution for the late-type population, R_eff ~ (1+z)^-0.75. The large sample size and dynamic range in both galaxy mass and redshift, in combination with the high fidelity of our measurements due to the extensive use of spectroscopic data, not only fortify previous results, but also enable us to probe beyond simple average galaxy size measurements. At all redshifts the slope of the size-mass relation is shallow, R_eff ~ M_star^0.22, for late-type galaxies with stellar mass >3x10^9 M_sol, and steep, R_eff M_star^0.75, for early-type galaxies with stellar mass >2x10^10 M_sol. The intrinsic scatter is <~0.2 dex for all galaxy types and redshifts. For late-type galaxies, the logarithmic size distribution is not symmetric, but skewed toward small sizes: at all redshifts and masses a tail of small late-type galaxies exists that overlaps in size with the early-type galaxy population. The number density of massive (~10^11 M_sol), compact (R_eff < 2 kpc) early-type galaxies increases from z=3 to z=1.5-2 and then strongly decreases at later cosmic times.

The SIMPLE survey: observations, reduction, and catalog

We present the Spitzer IRAC/MUSYC Public Legacy Survey in the Extended CDF-South (SIMPLE), which consists of deep IRAC observations covering the ~1,600 arcmin^2 area surrounding GOODS-S. The limiting magnitudes of the SIMPLE IRAC mosaics typically are 23.8, 23.6, 21.9, and 21.7, at 3.6 um, 4.5 um, 5.8 um, and 8.0 um, respectively (5-sigma total point source magnitudes in AB). The SIMPLE IRAC images are combined with the 10x15 GOODS IRAC mosaics in the center. We give detailed descriptions of the observations, data reduction, and properties of the final images, as well as the detection and photometry methods used to build a catalog. Using published optical and near-infrared data from the Multiwavelength Survey by Yale-Chile (MUSYC), we construct an IRAC-selected catalog, containing photometry in UBVRIzJHK, [3.6 um], [4.5 um], [5.8 um], and [8.0 um]. The catalog contains 43,782 sources with S/N > 5 at 3.6 um, 19,993 of which have 13-band photometry. We compare this catalog to the publicly available MUSYC and FIREWORKS catalogs and discuss the differences. Using a high signal-to-noise sub-sample of 3,391 sources with ([3.6] + [4.5])/2 < 21.2, we investigate the star formation rate history of massive galaxies out to z ~ 1.8. We find that at z ~ 1.8 at least 30% +/-7% of the most massive galaxies (Mstar > 10^11 Msol) are passively evolving, in agreement with earlier results from surveys covering less area.

Hubble Space Telescope Images of Red Mergers: How Dry are They?

Mergers between red galaxies are observed to be common in the nearby Universe, and are thought to be the dominant mechanism by which massive galaxies grow their mass at late times. These ``dry mergers can be readily identified in very deep ground based images, thanks to their extended low surface brightness tidal features. However, ground-based images lack the required resolution to determine the morphologies of the merging galaxies, and to measure the amount of dust and associated gas. We present HST/ACS and WFPC2 observations of a sample of 31 bulge-dominated red-sequence galaxies at z~0.1, comprised of ongoing mergers, merger remnants, and undisturbed galaxies. Nearly all galaxies have early-type morphologies and most are well-fit by r^1/4 law surface brightness profiles. We find that only 10% of the galaxies show evidence for the presence of dust. The amount of cold gas (or its upper limit) is calculated from the mean color-excess, assuming a simple relation between gas mass and dust mass. The gas mass is low for all galaxies, and we find that Mgas/Mstellar < 3x10^-4. We infer that red mergers in the nearby Universe mostly involve early-type galaxies containing little cold gas and dust. This may imply that the progenitors were mostly devoid of gas and/or that feedback mechanisms are very effective in preventing the gas to cool. The lack of gas in these objects may also imply a relatively large fraction of binary black holes in the centers of massive ellipticals.