We study the relationship between the structure and star-formation rate (SFR) of X-ray selected low and moderate luminosity active galactic nuclei (AGNs) in the two Chandra Deep Fields, using Hubble Space Telescope imaging from the Cosmic Assembly Ne
ar Infrared Extragalactic Legacy Survey (CANDELS) and deep far-infrared maps from the PEP+GOODS-Herschel survey. We derive detailed distributions of structural parameters and FIR luminosities from carefully constructed control samples of galaxies, which we then compare to those of the AGNs. At z~1, AGNs show slightly diskier light profiles than massive inactive (non-AGN) galaxies, as well as modestly higher levels of gross galaxy disturbance (as measured by visual signatures of interactions and clumpy structure). In contrast, at z~2, AGNs show similar levels of galaxy disturbance as inactive galaxies, but display a red central light enhancement, which may arise due to a more pronounced bulge in AGN hosts or due to extinguished nuclear light. We undertake a number of tests of these alternatives, but our results do not strongly favour one interpretation over the other. The mean SFR and its distribution among AGNs and inactive galaxies are similar at z>1.5. At z<1, however, clear and significant enhancements are seen in the SFRs of AGNs with bulge-dominated light profiles. These trends suggest an evolution in the relation between nuclear activity and host properties with redshift, towards a minor role for mergers and interactions at z>1.5.
We determine the intrinsic, 3-dimensional shape distribution of star-forming galaxies at 0<z<2.5, as inferred from their observed projected axis ratios. In the present-day universe star-forming galaxies of all masses 1e9 - 1e11 Msol are predominantly
thin, nearly oblate disks, in line with previous studies. We now extend this to higher redshifts, and find that among massive galaxies (M* > 1e10 Msol) disks are the most common geometric shape at all z < 2. Lower-mass galaxies at z>1 possess a broad range of geometric shapes: the fraction of elongated (prolate) galaxies increases toward higher redshifts and lower masses. Galaxies with stellar mass 1e9 Msol (1e10 Msol) are a mix of roughly equal numbers of elongated and disk galaxies at z~1 (z~2). This suggests that galaxies in this mass range do not yet have disks that are sustained over many orbital periods, implying that galaxies with present-day stellar mass comparable to that of the Milky Way typically first formed such sustained stellar disks at redshift z~1.5-2. Combined with constraints on the evolution of the star formation rate density and the distribution of star formation over galaxies with different masses, our findings imply that, averaged over cosmic time, the majority of stars formed in disks.
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 rang
e 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.
Using spectroscopy from the Large Binocular Telescope and imaging from the Hubble Space Telescope we discovered the first strong galaxy lens at z(lens)>1. The lens has a secure photometric redshift of z=1.53+/-0.09 and the source is spectroscopically
confirmed at z=3.417. The Einstein radius (0.35; 3.0 kpc) encloses 7.6 x 10^10 Msol, with an upper limit on the dark matter fraction of 60%. The highly magnified (40x) source galaxy has a very small stellar mass (~10^8 Msol) and shows an extremely strong [OIII]_5007A emission line (EW_0 ~ 1000A) bolstering the evidence that intense starbursts among very low-mass galaxies are common at high redshift.
We present global structural parameter measurements of 109,533 unique, H_F160W-selected objects from the CANDELS multi-cycle treasury program. Sersic model fits for these objects are produced with GALFIT in all available near-infrared filters (H_F160
W, J_F125W and, for a subset, Y_F105W). The parameters of the best-fitting Sersic models (total magnitude, half-light radius, Sersic index, axis ratio, and position angle) are made public, along with newly constructed point spread functions for each field and filter. Random uncertainties in the measured parameters are estimated for each individual object based on a comparison between multiple, independent measurements of the same set of objects. To quantify systematic uncertainties we create a mosaic with simulated galaxy images with a realistic distribution of input parameters and then process and analyze the mosaic in an identical manner as the real data. We find that accurate and precise measurements -- to 10% or better -- of all structural parameters can typically be obtained for galaxies with H_F160W < 23, with comparable fidelity for basic size and shape measurements for galaxies to H_F160W ~ 24.5.
We identify an abundant population of extreme emission line galaxies (EELGs) at redshift z~1.7 in the Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey (CANDELS) imaging from Hubble Space Telescope/Wide Field Camera 3 (HST/WFC3). 69 EELG candi
dates are selected by the large contribution of exceptionally bright emission lines to their near-infrared broad-band magnitudes. Supported by spectroscopic confirmation of strong [OIII] emission lines -- with rest-frame equivalent widths ~1000AA -- in the four candidates that have HST/WFC3 grism observations, we conclude that these objects are galaxies with 10^8 Msol in stellar mass, undergoing an enormous starburst phase with M_*/(dM_*/dt) of only ~15 Myr. These bursts may cause outflows that are strong enough to produce cored dark matter profiles in low-mass galaxies. The individual star formation rates and the co-moving number density (3.7x10^-4 Mpc^-3) can produce in ~4 Gyr much of the stellar mass density that is presently contained in 10^8-10^9 Msol dwarf galaxies. Therefore, our observations provide a strong indication that many or even most of the stars in present-day dwarf galaxies formed in strong, short-lived bursts, mostly at z>1.
The Sloan Digital Sky Survey (SDSS) and photometric/spectroscopic surveys in the GOODS-South field (the Chandra Deep Field-South, CDFS) are used to construct volume-limited, stellar mass-selected samples of galaxies at redshifts 0<z<1. The CDFS sampl
e at 0.6<z<1.0 contains 207 galaxies complete down to M=4x10^10 Msol (for a ``diet Salpeter IMF), corresponding to a luminosity limit for red galaxies of M_B=-20.1. The SDSS sample at 0.020<z<0.045 contains 2003 galaxies down to the same mass limit, which corresponds to M_B=-19.3 for red galaxies. Morphologies are determined with an automated method, using the Sersic parameter n and a measure of the residual from the model fits, called ``bumpiness, to distinguish different morphologies. These classifications are verified with visual classifications. In agreement with previous studies, 65-70% of the galaxies are located on the red sequence, both at z~0.03 and at z~0.8. Similarly, 65-70% of the galaxies have n>2.5. The fraction of E+S0 galaxies is 43+/-3%$ at z~0.03 and 48+/-7% at z~0.8, i.e., it has not changed significantly since z~0.8. When combined with recent results for cluster galaxies in the same redshift range, we find that the morphology-density relation for galaxies more massive than 0.5M* has remained constant since at least z~0.8. This implies that galaxies evolve in mass, morphology and density such that the morphology-density relation does not change. In particular, the decline of star formation activity and the accompanying increase in the stellar mass density of red galaxies since z~1 must happen without large changes in the early-type galaxy fraction in a given environment.
