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 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.

We study the evolution of the scaling relations that compare the effective density ( r r , e e S < ) and core density ( r , 1 1 S < kpc) to the stellar masses of star-forming galaxies (SFGs) and quiescent galaxies. These relations have been fully in place since z 3 and have exhibited almost constant slope and scatter since that time. For SFGs, the zero points in e S and 1 S decline by only 2 . This fact plus the narrowness of the relations suggests that galaxies could evolve roughly along the scaling relations. Quiescent galaxies follow different scaling relations that are offset to higher densities at the same mass and redshift. Furthermore, the zero point of their core density has declined by only 2 since z 3 , while the zero point of the effective density declines by 10 . When galaxies quench, they move from the star-forming relations to the quiescent relations. This involves an increase in the core and effective densities, which suggests that SFGs could experience a phase of significant core growth relative to the average evolution along the structural relations. The distribution of massive galaxies relative to the SFR-M and the quiescent M  Srelations exhibits an L-shape that is independent of redshift. The knee of this relation consists of a subset of "compact" SFGs that are the most likely precursors of quiescent galaxies forming at later times. The compactness selection threshold in 1 S exhibits a small variation from z=3 to 0.5, M 0.65 log 10.5 9.6 9.3 1 * S --> -( ) M e kpc −2 , allowing the most efficient identification of compact SFGs and quiescent galaxies at every redshift.

We present a systematic study of the shape of the dust attenuation curve in star‐forming galaxies from the far‐ultraviolet (far‐UV) to the near‐infrared (NIR; ∼0.15–2 μ m), as a function of specific star formation rate (ψS) and axial ratio (b/a), for galaxies with and without a significant bulge. Our sample comprises 23 000 (15 000) galaxies with a median redshift of 0.07, with photometric entries in the Sloan Digital Sky Survey (SDSS), UKIRT Infrared Deep Sky Survey‐Large Area Survey and Galaxy Evolution Explorer‐All‐Sky Imaging Survey catalogues and emission‐line measurements from the SDSS spectroscopic survey. We develop a new pair‐matching technique to isolate the dust attenuation curves from the stellar continuum emission. The main results are: (i) the slope of the attenuation curve in the optical varies weakly with ψS, strongly with b/a, and is significantly steeper than the Milky Way extinction law in bulge‐dominated galaxies; (ii) the NIR slope is constant and matches the slope of the Milky Way extinction law; (iii) the UV has a slope change consistent with a dust bump at 2175 Å which is evident in all samples and varies strongly in strength with b/a in the bulge‐dominated sample; (iv) there is a strong increase in emission‐line‐to‐continuum dust attenuation (τV, line/τV, cont) with both decreasing ψS and increasing b/a; and (v) radial gradients in dust attenuation increase strongly with increasing ψS, and the presence of a bulge does not alter the strength of the gradients. These results are consistent with the picture in which young stars are surrounded by dense ‘birth clouds’ with low covering factor which disperse on time‐scales of ∼107 yr and the diffuse interstellar dust is distributed in a centrally concentrated disc with a smaller scaleheight than the older stars that contribute the majority of the red and NIR light. Within this model, the path‐length of diffuse dust, but not of birth‐cloud dust, increases with increasing inclination and the apparent optical attenuation curve is steepened by the differential effect of larger dust opacity towards younger stars than towards older stars. Additionally, our findings suggest that: (i) galaxies with higher star formation rates per unit stellar mass have a higher fraction of diffuse dust, which is more centrally concentrated; (ii) the observed strength of the 2175‐Å dust feature is affected predominantly by global geometry; and (iii) only highly inclined discs are optically thick. We provide new empirically derived attenuation curves for correcting the light from star‐forming galaxies for dust attenuation.

Emission line diagnostic diagrams probing the ionization sources in galaxies, such as the Baldwin-Phillips-Terlevich (BPT) diagram, have been used extensively to distinguish AGN from purely starforming galaxies. Yet, they remain poorly understood at higher redshifts. We shed light on this issue with an empirical approach based on a z ∼ 0 reference sample built from ∼300,000 SDSS galaxies, from which we mimic selection effects due to typical emission line detection limits at higher redshift. We combine this low-redshift reference sample with a simple prescription for luminosity evolution of the global galaxy population to predict the loci of high-redshift galaxies on the BPT and Mass-Excitation (MEx) diagnostic diagrams. The predicted bivariate distributions agree remarkably well with direct observations of galaxies out to z ∼ 1.5, including the observed stellar mass-metallicity (M Z) relation evolution. As a result, we infer that high-redshift star-forming galaxies are consistent with having normal ISM properties out to z ∼ 1.5, after accounting for selection effects and line luminosity evolution. Namely, their optical line ratios and gas-phase metallicities are comparable to that of low-redshift galaxies with equivalent emission-line luminosities. In contrast, AGN narrow-line regions may show a shift toward lower metallicities at higher redshift. While a physical evolution of the ISM conditions is not ruled out for purely star-forming galaxies, and may be more important starting at z 2, we find that reliably quantifying this evolution is hindered by selections effects. The recipes provided here may serve as a basis for future studies toward this goal. Code to predict the loci of galaxies on the BPT and MEx diagnostic diagrams, and the M Z relation as a function of emission line luminosity limits, is made publicly available. High-redshift galaxy samplesThe intermediate to high redshift galaxies used in this work were selected from the following:• 0.3 < z < 1 galaxies with R AB < 24.3 and < 24.1 from the TKRS and DEEP2 redshift surveys, respectively (J11);• z ∼ 1.4 galaxies with K < 23.9, 1.2 < z phot < 1.6, M ⋆ > 10 9.5 M ⊙ from the SXDS/UDS 12 fields with NIR spectra (Yabe et al. 2012, hereafter Y12);• z ∼ 1.5 emission-line selected galaxies from the GOODS-S field with NIR spectra (Trump et al. 2013, hereafter T13);• z ∼ 2 galaxies from the SINS/zC-SINF survey, GOODS-N and Q2343 fields, with NIR spectra, and selected by Newman et al. (2014, hereafter N14).

Dust attenuation affects nearly all observational aspects of galaxy evolution, yet very little is known about the form of the dust-attenuation law in the distant Universe. Here, we model the spectral energy distributions (SEDs) of galaxies at z ∼ 1.5-3 from CANDELS with rest-frame UV to near-IR imaging under different assumptions about the dust law, and compare the amount of inferred attenuated light with the observed infrared (IR) luminosities. Some individual galaxies show strong Bayesian evidence in preference of one dust law over another, and this preference agrees with their observed location on the plane of infrared excess (IRX, L TIR /L UV ) and UV slope (β). We generalize the shape of the dust law with an empirical model, Calzetti et al. (2000), and show that there exists a correlation between the color excess E(B − V ) and tilt δ with δ = (0.62 ± 0.05) log(E(B − V ))+(0.26 ± 0.02). Galaxies with high color excess have a shallower, starburst-like law, and those with low color excess have a steeper, SMC-like law. Surprisingly, the galaxies in our sample show no correlation between the shape of the dust law and stellar mass, star-formation rate, or β. The change in the dust law with color excess is consistent with a model where attenuation is caused by by scattering, a mixed star-dust geometry, and/or trends with stellar population age, metallicity, and dust grain size. This rest-frame UV-to-near-IR method shows potential to constrain the dust law at even higher (z > 3) redshifts.

We present near-infrared spectroscopy of a sample of 22 Extreme Emission Line Galaxies at redshifts 1.3 < z < 2.3, confirming that these are low-mass (M ⋆ = 10 8 − 10 9 M ⊙ ) galaxies undergoing intense starburst episodes (M ⋆ /SF R ∼ 10 − 100 Myr). The sample is selected by [O III] or Hα emission line flux and equivalent width using near-infrared grism spectroscopy from the 3D-HST survey. Highresolution NIR spectroscopy is obtained with LBT/LUCI and VLT/X-SHOOTER. The [O III]/Hβ line ratio is high ( 5) and [N II]/Hα is always significantly below unity, which suggests a low gasphase metallicity. We are able to determine gas-phase metallicities for 7 of our objects using various strong-line methods, with values in the range 0.05 − 0.30 Z ⊙ and with a median of 0.15 Z ⊙ ; for 3 of these objects we detect [O III]λ4363 which allows for a direct constraint on the metallicity. The velocity dispersion, as measured from the nebular emission lines, is typically ∼ 50 km s −1 . Combined with the observed star-forming activity, the Jeans and Toomre stability criteria imply that the gas fraction must be large (f gas 2/3), consistent with the difference between our dynamical and stellar mass estimates. The implied gas depletion time scale (several hundred Myr) is substantially longer than the inferred mass-weighted ages (∼50 Myr), which further supports the emerging picture that most stars in low-mass galaxies form in short, intense bursts of star formation.

We present a new approach to constrain galaxy physical parameters from the combined interpretation of stellar and nebular emission in wide ranges of observations. This approach relies on the Bayesian analysis of any type of galaxy spectral energy distribution using a comprehensive library of synthetic spectra assembled using state‐of‐the‐art models of star formation and chemical enrichment histories, stellar population synthesis, nebular emission and attenuation by dust. We focus on the constraints set by five‐band ugriz photometry and low‐ and medium‐resolution spectroscopy at rest wavelengths λ= 3600–7400 Å on a few physical parameters of galaxies: the observer‐frame absolute r‐band stellar mass‐to‐light ratio, M*/Lr; the fraction of a current galaxy stellar mass formed during the last 2.5 Gyr, fSFH; the specific star formation rate, ψS; the gas‐phase oxygen abundance, 12 + log(O/H); the total effective V‐band absorption optical depth of the dust, ; and the fraction of this arising from dust in the ambient interstellar medium, μ. Since these parameters cannot be known a priori for any galaxy sample, we assess the accuracy to which they can be retrieved from observations by simulating ‘pseudo‐observations’ using models with known parameters. Assuming that these models are good approximations of true galaxies, we find that the combined analysis of stellar and nebular emission in low‐resolution [50 Å full width at half‐maximum (FWHM)] galaxy spectra provides valuable constraints on all physical parameters. The typical uncertainties in high‐quality spectra are about 0.13 dex for M*/Lr, 0.23 for fSFH, 0.24 dex for ψS, 0.28 for 12 + log(O/H), 0.64 for and 0.16 for μ. The uncertainties in 12 + log(O/H) and tighten by about 20 per cent for galaxies with detectable emission lines and by another 45 per cent when the spectral resolution is increased to 5 Å FWHM. At this spectral resolution, the analysis of the combined stellar and nebular emission in the high‐quality spectra of 12 660 Sloan Digital Sky Survey (SDSS) star‐forming galaxies using our approach yields likelihood distributions of M★, 12 + log(O/H), and ψS similar to those obtained in previous separate analyses of the stellar and nebular emission at the original (twice higher) SDSS spectral resolution. Meanwhile, rest‐frame ugriz photometry provides competitive constraints on M*/Lr. We show that the constraints derived on galaxy physical parameters from these different types of observations depend sensitively on signal‐to‐noise ratio. Our approach can be extended to the analysis of any type of observation across the wavelength range covered by spectral evolution models.