The remarkable Hubble Space Telescope(HST) data sets from the CANDELS, HUDF09, HUDF12, ERS, and BoRG/HIPPIES programs have allowed us to map the evolution of the rest-frame UV luminosity function (LF) fromz 10 toz 4. We develop new color criteria that more optimally utilize the full wavelength coverage from the optical, near-IR, and mid-IR observations over our search fields, while simultaneously minimizing the incompleteness and eliminating redshift gaps. We have identified 5859, 3001, 857, 481, 217, and 6 galaxy candidates atz 4,z 5,z 6,z 7,z 8, andz 10, respectively, from the ∼1000 arcmin 2 area covered by these data sets. This sample of >10,000 galaxy candidates at ⩾ z 4 is by far the largest assembled to date with HST. The selection ofz 4-8 candidates over the five CANDELS fields allows us to assess the cosmic variance; the largest variations are at ⩾ z 7. Our new LF determinations atz 4 andz 5 span a 6 mag baseline and reach to -16 AB mag. These determinations agree well with previous estimates, but the larger samples and volumes probed here result in a more reliable sampling of >L* galaxies and allow us to reassess the form of the UV LFs. Our new LF results strengthen our earlier findings to s 3.4 significance for a steeper faint-end slope of the UV LF at > z 4, with α evolving from a = - 1.64 0.04 atz 4 to a = - 2.06 0.13 atz 7 (and a = - 2.02 0.23 atz 8), consistent with that expected from the evolution of the halo mass function. We find less evolution in the characteristic magnitude M * fromz 7 toz 4; the observed evolution in the LF is now largely represented by changes in f*. No evidence for a non-Schechter-like form to the z ∼ 4-8 LFs is found. A simple conditional LF model based on halo growth and evolution in the M/L ratio µ +z ( ( 1) ) 1.5 of halos provides a good representation of the observed evolution.

We constrain the slope of the star formation rate (log Ψ) to stellar mass (log M ⋆ ) relation down to log(M ⋆ /M ⊙ ) = 8.4 (log(M ⋆ /M ⊙ ) = 9.2) at z = 0.5 (z = 2.5) with a mass-complete sample of 39,106 star-forming galaxies selected from the 3D-HST photometric catalogs, using deep photometry in the CANDELS fields. For the first time, we find that the slope is dependent on stellar mass, such that it is steeper at low masses (log Ψ ∝ log M ⋆ ) than at high masses (log Ψ ∝ (0.3 − 0.6) log M ⋆ ). These steeper low mass slopes are found for three different star formation indicators: the combination of the ultraviolet (UV) and infrared (IR), calibrated from a stacking analysis of Spitzer/MIPS 24µm imaging; β-corrected UV SFRs; and Hα SFRs. The normalization of the sequence evolves differently in distinct mass regimes as well: for galaxies less massive than log(M ⋆ /M ⊙ ) < 10 the specific SFR (Ψ/M ⋆ ) is observed to be roughly self-similar with Ψ/M ⋆ ∝ (1 + z) 1.9 , whereas more massive galaxies show a stronger evolution with Ψ/M ⋆ ∝ (1 + z) 2.2−3.5 for log(M ⋆ /M ⊙ ) = 10.2 − 11.2. The fact that we find a steep slope of the star formation sequence for the lower mass galaxies will help reconcile theoretical galaxy formation models with the observations.

We investigate the properties of quiescent and star-forming galaxy populations to z ∼ 2 with purely photometric data, employing a novel rest-frame color-selection technique. From the UKIDSS Ultra-Deep Survey Data Release 1, with matched optical and mid-infrared photometry taken from the Subaru-XMM Deep Survey and Spitzer Wide-Area Infrared Extragalactic Survey, respectively, we construct a K-selected galaxy catalog and calculate photometric redshifts. Excluding stars, objects with uncertain z phot solutions, those that fall in bad or incomplete survey regions, and those for which reliable rest-frame colors could not be derived, 30,108 galaxies with K < 22.4 (AB) and z phot 2.5 remain. The galaxies in this sample are found to occupy two distinct populations in the rest-frame U−V versus V−J color space: a clump of red, quiescent galaxies (analogous to the red sequence) and a track of star-forming galaxies extending from blue to red U−V colors. This bimodal behavior is seen up to z ∼ 2. Due to a combination of measurement errors and passive evolution, the color-color diagram is not suitable to distinguish the galaxy bimodality at z > 2 for this sample, but we show that MIPS 24 μm data suggest that a significant population of quiescent galaxies exists even at these higher redshifts. At z = 1-2, the most luminous objects in the sample are divided roughly equally between star-forming and quiescent galaxies, while at lower redshifts most of the brightest galaxies are quiescent. Moreover, quiescent galaxies at these redshifts are clustered more strongly than those actively forming stars, indicating that galaxies with early-quenched star formation may occupy more massive host dark matter halos. This suggests that the end of star formation is associated with, and perhaps brought about by, a mechanism related to halo mass.

We present measurements of the stellar mass functions (SMFs) of star-forming and quiescent galaxies to z = 4 using a sample of 95 675 galaxies in the COSMOS/UltraVISTA field. Sources have been selected from the DR1 UltraVISTA K s -band imaging which covers a unique combination of a wide area (1.62 deg 2 ), to a significant depth (K s,tot = 23.4, 90% completeness). The SMFs of the combined population are in good agreement with previous measurements and show that the stellar mass density of the universe was only 50%, 10% and 1% of its current value at z ∼ 0.75, 2.0, and 3.5, respectively. The quiescent population drives most of the overall growth, with the stellar mass density of these galaxies increasing as ρ star ∝ (1 + z) −4.7±0.4 since z = 3.5, whereas the mass density of star-forming galaxies increases as ρ star ∝ (1 + z) −2.3±0.2 . At z > 2.5, star-forming galaxies dominate the total SMF at all stellar masses, although a nonzero population of quiescent galaxies persists to z = 4. Comparisons of the K s -selected star-forming galaxy SMFs to UV-selected SMFs at 2.5 < z < 4 show reasonable agreement and suggests UV-selected samples are representative of the majority of the stellar mass density at z > 3.5. We estimate the average mass growth of individual galaxies by selecting galaxies at fixed cumulative number density. The average galaxy with Log(M * /M ⊙ ) = 11.5 at z = 0.3 has grown in mass by only 0.2 dex (0.3 dex) since z = 2.0(3.5), whereas those with Log(M * /M ⊙ ) = 10.5 have grown by > 1.0 dex since z = 2. At z < 2, the time derivatives of the mass growth are always larger for lower-mass galaxies, which demonstrates that the mass growth in galaxies since that redshift is mass-dependent and primarily bottom-up. Lastly, we examine potential sources of systematic uncertainties on the SMFs and find that those from photo-z templates, SPS modeling, and the definition of quiescent galaxies dominate the total error budget in the SMFs.

The 3D-HST and CANDELS programs have provided WFC3 and ACS spectroscopy and photometry over ≈ 900 arcmin 2 in five fields: AEGIS, COSMOS, GOODS-North, GOODS-South, and the UKIDSS UDS field. All these fields have a wealth of publicly available imaging datasets in addition to the HST data, which makes it possible to construct the spectral energy distributions (SEDs) of objects over a wide wavelength range. In this paper we describe a photometric analysis of the CANDELS and 3D-HST HST imaging and the ancillary imaging data at wavelengths 0.3 µm -8 µm. Objects were selected in the WFC3 near-IR bands, and their SEDs were determined by carefully taking the effects of the point spread function in each observation into account. A total of 147 distinct imaging datasets were used in the analysis. The photometry is made available in the form of six catalogs: one for each field, as well as a master catalog containing all objects in the entire survey. We also provide derived data products: photometric redshifts, determined with the EAZY code, and stellar population parameters determined with the FAST code. We make all the imaging data that were used in the analysis available, including our reductions of the WFC3 imaging in all five fields. 3D-HST is a spectroscopic survey with the WFC3 and ACS grisms, and the photometric catalogs presented here constitute a necessary first step in the analysis of these grism data. All the data presented in this paper are available through the 3D-HST website. 16