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.

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

We study the growth of massive galaxies from z = 2 to the present using data from the NEWFIRM Medium Band Survey (NMBS). The sample is selected at a constant number density of n = 2 × 10 −4 Mpc −3 , so that galaxies at different epochs can be compared in a meaningful way. We show that the stellar mass of galaxies at this number density has increased by a factor of ≈ 2 since z = 2, following the relation log M n (z) = 11.45 − 0.15z. In order to determine at what physical radii this mass growth occurred we construct very deep stacked restframe R band images of galaxies with masses near M n (z), at redshifts z = 0.6, 1.1, 1.6, and 2.0. These image stacks of typically 70-80 galaxies enable us to characterize the stellar distribution to surface brightness limits of ∼ 28.5 mag arcsec −2 . We find that massive galaxies gradually built up their outer regions over the past 10 Gyr. The mass within a radius of r = 5 kpc is nearly constant with redshift whereas the mass at 5 kpc < r < 75 kpc has increased by a factor of ∼ 4 since z = 2. Parameterizing the surface brightness profiles we find that the effective radius and Sersic n parameter evolve as r e ∝ (1 + z) −1.3 and n ∝ (1 + z) −1.0 respectively. The data demonstrate that massive galaxies have grown mostly inside-out, assembling their extended stellar halos around compact, dense cores with possibly exponential radial density distributions. Comparing the observed mass evolution to the average star formation rates of the galaxies we find that the growth is likely dominated by mergers, as insitu star formation can only account for ∼ 20 % of the mass build-up from z = 2 to z = 0. A direct consequence of these results is that massive galaxies do not evolve in a self-similar way: their structural profiles change as a function of redshift, complicating analyses which (often implicitly) assume self-similarity. The main uncertainties in this study are possible redshift-dependent systematic errors in the total stellar masses and the conversion from light-weighted to mass-weighted radial profiles.

We present reduced data and data products from the 3D-HST survey, a 248-orbit HST Treasury program. The survey obtained WFC3 G141 grism spectroscopy in four of the five CANDELS fields: AEGIS, COSMOS, GOODS-S, and UDS, along with WFC3 H 140 imaging, parallel ACS G800L spectroscopy, and parallel I 814 imaging. In a previous paper, we presented photometric catalogs in these four fields and in GOODS-N, the fifth CANDELS field. Here we describe and present the WFC3 G141 spectroscopic data, again augmented with data from GO-1600 in GOODS-N (PI: B. Weiner). We developed software to automatically and optimally extract interlaced two-dimensional (2D) and one-dimensional (1D) spectra for all objects in the Skelton et al. (2014) photometric catalogs. The 2D spectra and the multi-band photometry were fit simultaneously to determine redshifts and emission line strengths, taking the morphology of the galaxies explicitly into account. The resulting catalog has redshifts and line strengths (where available) for 22,548 unique objects down to  JH 24 IR (79,609 unique objects down to  JH 26 IR ). Of these, 5459 galaxies are at > z 1.5 and 9621 are at < < z 0.7 1.5, where Hα falls in the G141 wavelength coverage. The typical redshift error for  JH 24 IR galaxies is s »´+z 0.003 1 z ( ), i.e., one native WFC3 pixel. The s 3 limit for emission line fluxes of point sources is´-2.1 10 17 erg s −1 cm −2 . All 2D and 1D spectra, as well as redshifts, line fluxes, and other derived parameters, are publicly available.

The Astropy Project supports and fosters the development of open-source and openly developed Python packages that provide commonly needed functionality to the astronomical community. A key element of the Astropy Project is the core package astropy, which serves as the foundation for more specialized projects and packages. In this article, we provide an overview of the organization of the Astropy project and summarize key features in the core package, as of the recent major release, version 2.0. We then describe the project infrastructure designed to facilitate and support development for a broader ecosystem of interoperable packages. We conclude with a future outlook of planned new features and directions for the broader Astropy Project.

In this paper we study a key phase in the formation of massive galaxies: the transition of star forming galaxies into massive (M stars ∼ 10 11 M ⊙ ), compact (r e ∼ 1 kpc) quiescent galaxies, which takes place from z ∼ 3 to z ∼ 1.5. We use HST grism redshifts and extensive photometry in all five 3D-HST/CANDELS fields, more than doubling the area used previously for such studies, and combine these data with Keck MOSFIRE and NIRSPEC spectroscopy. We first confirm that a population of massive, compact, star forming galaxies exists at z 2, using K-band spectroscopy of 25 of these objects at 2.0 < z < 2.5. They have a median [NII]/Hα ratio of 0.6, are highly obscured with SFR(tot)/SFR(Hα) ∼ 10, and have a large range of observed line widths. We infer from the kinematics and spatial distribution of Hα that the galaxies have rotating disks of ionized gas that are a factor of ∼ 2 more extended than the stellar distribution. By combining measurements of individual galaxies, we find that the kinematics are consistent with a nearly Keplerian fall-off from V rot ∼ 500 km s −1 at 1 kpc to V rot ∼ 250 km s −1 at 7 kpc, and that the total mass out to this radius is dominated by the dense stellar component. Next, we study the size and mass evolution of the progenitors of compact massive galaxies. Even though individual galaxies may have had complex histories with periods of compaction and mergers, we show that the population of progenitors likely followed a simple inside-out growth track in the size-mass plane of ∆ log r e ∼ 0.3∆ log M stars . This mode of growth gradually increases the stellar mass within a fixed physical radius, and galaxies quench when they reach a stellar density or velocity dispersion threshold. As shown in other studies, the mode of growth changes after quenching, as dry mergers take the galaxies on a relatively steep track in the size-mass plane.

Galaxy observations are influenced by many physical parameters: stellar masses, star formation rates (SFRs), star formation histories (SFHs), metallicities, dust, black hole activity, and more. As a result, inferring accurate physical parameters requires high-dimensional models which capture or marginalize over this complexity. Here we re-assess inferences of galaxy stellar masses and SFRs using the 14-parameter physical model Prospector-α built in the Prospector Bayesian inference framework. We fit the photometry of 58,461 galaxies from the 3D-HST catalogs at 0.5 < z < 2.5. The resulting stellar masses are ∼ 0.1 − 0.3 dex larger than the fiducial masses while remaining consistent with dynamical constraints. This change is primarily due to the systematically older SFHs inferred with Prospector. The SFRs are ∼ 0.1 − 1+ dex lower than UV+IR SFRs, with the largest offsets caused by emission from "old" (t > 100 Myr) stars. These new inferences lower the observed cosmic star formation rate density by ∼ 0.2 dex and increase the observed stellar mass growth by ∼ 0.1 dex, finally bringing these two quantities into agreement and implying an older, more quiescent Universe than found by previous studies at these redshifts. We corroborate these results by showing that the Prospector-α SFHs are both more physically realistic and are much better predictors of the evolution of the stellar mass function. Finally, we highlight examples of observational data which can break degeneracies in the current model; these observations can be incorporated into priors in future models to produce new & more accurate physical parameters.

We study the growth of the red sequence through the number density and structural evolution of a sample of young and old quiescent galaxies at 0 < z < 2. The galaxies are selected from the NEWFIRM Medium-Band Survey (NMBS) in the Cosmic Evolution Survey (COSMOS) field. We find a large population of massive young recently quenched ("post-starburst") galaxies at z > 1 that are almost non-existent at z < 1; their number density is 5×10 −5 Mpc −3 at z=2, whereas it is a factor of 10 less at z = 0.5. The observed number densities of young and old quiescent galaxies at z > 1 are consistent with a simple model in which all old quiescent galaxies were once identified as post-starburst galaxies. We find that the overall population of quiescent galaxies have smaller sizes and slightly more elongated shapes at higher redshift, in agreement with other recent studies. Interestingly, the most recently quenched galaxies at 1 < z < 2 are not larger, and possibly even smaller, than older galaxies at those redshifts. This result is inconsistent with the idea that the evolution of the average size of quiescent galaxies is largely driven by continuous transformations of larger, star-forming galaxies: in that case, the youngest quiescent galaxies would also be the largest. Instead, mergers or other mechanisms appear to be required to explain the size growth of quiescent galaxies from z = 2 to the present.