The FourStar galaxy evolution survey (ZFOURGE) is a 45 night legacy program with the FourStar near-infrared camera on Magellan and one of the most sensitive surveys to date. ZFOURGE covers a total of 400 arcmin² in cosmic fields CDFS, COSMOS and UDS, overlapping CANDELS. We present photometric catalogs comprising >70,000 galaxies, selected from ultradeep K _s -band detection images (25.5–26.5 AB mag, 5σ, total), and >80% complete to K _s < 25.3–25.9 AB. We use 5 near-IR medium-bandwidth filters (J ₁, J ₂, J ₃, H _s , H _l ) as well as broad-band K _s at 1.05–2.16 μm to 25–26 AB at a seeing of ∼0.″5. Each field has ancillary imaging in 26–40 filters at 0.3–8 μm. We derive photometric redshifts and stellar population properties. Comparing with spectroscopic redshifts indicates a photometric redshift uncertainty σ _z = 0.010, 0.009, and 0.011 in CDFS, COSMOS, and UDS. As spectroscopic samples are often biased toward bright and blue sources, we also inspect the photometric redshift differences between close pairs of galaxies, finding σ _z,pairs = 0.01–0.02 at 1 < z < 2.5. We quantify how σ _z,pairs depends on redshift, magnitude, spectral energy distribution type, and the inclusion of FourStar medium bands. σ _z,pairs is smallest for bright, blue star-forming samples, while red star-forming galaxies have the worst σ _z,pairs. Including FourStar medium bands reduces σ _z,pairs by 50% at 1.5 < z < 2.5. We calculate star formation rates (SFRs) based on ultraviolet and ultradeep far-IR Spitzer/MIPS and Herschel/PACS data. We derive rest-frame U − V and V − J colors, and illustrate how these correlate with specific SFR and dust emission to z = 3.5. We confirm the existence of quiescent galaxies at z ∼ 3, demonstrating their SFRs are suppressed by > ×15.

We study galactic star formation activity as a function of environment and stellar mass over 0.5 < z < 2.0 using the FourStar Galaxy Evolution (ZFOURGE) survey. We estimate the galaxy environment using a Bayesian-motivated measure of the distance to the third nearest neighbor for galaxies to the stellar mass completeness of our survey, at z = 1.3 (2.0). This method, when applied to a mock catalog with the photometric-redshift precision ( ) of ZFOURGE, accurately recovers galaxies in low- and high-density environments. We quantify the environmental quenching efficiency and show that at , it depends on galaxy stellar mass, demonstrating that the effects of quenching related to (stellar) mass and environment are not separable. In high-density environments, the mass and environmental quenching efficiencies are comparable for massive galaxies ( ) at all redshifts. For lower-mass galaxies ( ), the environmental quenching efficiency is very low at , but increases rapidly with decreasing redshift. Environmental quenching can account for nearly all quiescent lower-mass galaxies ( ), which appear primarily at . The morphologies of lower-mass quiescent galaxies are inconsistent with those expected of recently quenched star-forming galaxies. Some environmental process must transform the morphologies on similar timescales as the environmental quenching itself. The evolution of the environmental quenching favors models that combine gas starvation (as galaxies become satellites) with gas exhaustion through star formation and outflows (“overconsumption”), and additional processes such as galaxy interactions, tidal stripping, and disk fading to account for the morphological differences between the quiescent and star-forming galaxy populations.

We present an overview and the first data release of ZFIRE, a spectroscopic redshift survey of star-forming galaxies that utilizes the MOSFIRE instrument on Keck-I to study galaxy properties in rich environments at 1.5 < z < 2.5. ZFIRE measures accurate spectroscopic redshifts and basic galaxy properties derived from multiple emission lines. The galaxies are selected from a stellar mass limited sample based on deep near infrared imaging ( ) and precise photometric redshifts from the ZFOURGE and UKIDSS surveys as well as grism redshifts from 3DHST. Between 2013 and 2015, ZFIRE has observed the COSMOS and UDS legacy fields over 13 nights and has obtained 211 galaxy redshifts over 1.57 < z < 2.66 from a combination of nebular emission lines (such as Hα, [N ii], Hβ, [O ii], [O iii], and [S ii]) observed at 1–2 μm. Based on our medium-band near infrared photometry, we are able to spectrophotometrically flux calibrate our spectra to ∼10% accuracy. ZFIRE reaches 5σ emission line flux limits of ∼3 × 10⁻¹⁸ erg s⁻¹ cm⁻² with a resolving power of R = 3500 and reaches masses down to ∼10⁹ M _⊙. We confirm that the primary input survey, ZFOURGE, has produced photometric redshifts for star-forming galaxies (including highly attenuated ones) accurate to with 0.7% outliers. We measure a slight redshift bias of <0.001, and we note that the redshift bias tends to be larger at higher masses. We also examine the role of redshift on the derivation of rest-frame colors and stellar population parameters from SED fitting techniques. The ZFIRE survey extends spectroscopically confirmed z ∼ 2 samples across a richer range of environments, here we make available the first public release of the data for use by the community.⁷

For the first time, we present the size evolution of a mass-complete (log(M _*/M _⊙) > 10) sample of star-forming galaxies over redshifts z = 1–7, selected from the FourStar Galaxy Evolution Survey. Observed H-band sizes are measured from the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) Hubble Space Telescope (HST)/F160W imaging. Distributions of individual galaxy masses and sizes illustrate that a clear mass−size relation exists up to z ∼ 7. At z ∼ 7, we find that the average galaxy size from the mass−size relation is more compact at a fixed mass of log(M _*/M _⊙) = 10.1, with kpc, than at lower redshifts. This is consistent with our results from stacking the same CANDELS HST/F160W imaging, when we correct for galaxy position angle alignment. We find that the size evolution of star-forming galaxies is well fit by a power law of the form kpc, which is consistent with previous works for normal star-formers at 1 < z < 4. In order to compare our slope with those derived Lyman break galaxy studies, we correct for different IMFs and methodology and find a slope of −0.97 ± 0.02, which is shallower than that reported for the evolution of Lyman break galaxies (LBGs) at z > 4 ( ). Therefore, we conclude the LBGs likely represent a subset of highly star-forming galaxies that exhibit rapid size growth at z > 4.

We investigate the star formation rate (SFR) dependence on the stellar mass and gas-phase metallicity relation at z = 2 with MOSFIRE/Keck as part of the ZFIRE survey. We have identified 117 galaxies (1.98 ≤ z ≤ 2.56), with 8.9 ≤ log(M/M _⊙) ≤ 11.0, for which we can measure gas-phase metallicities. For the first time, we show a discernible difference between the mass–metallicity relation, using individual galaxies, when dividing the sample by low (10 M _⊙ yr⁻¹) SFRs. At fixed mass, low star-forming galaxies tend to have higher metallicity than high star-forming galaxies. Using a few basic assumptions, we further show that the gas masses and metallicities required to produce the fundamental mass–metallicity relation and its intrinsic scatter are consistent with cold-mode accretion predictions obtained from the OWLS hydrodynamical simulations. Our results from both simulations and observations are suggestive that cold-mode accretion is responsible for the fundamental mass–metallicity relation at z = 2 and it demonstrates the direct relationship between cosmological accretion and the fundamental properties of galaxies.

Using deep multi-wavelength photometry of galaxies from zfourge, we group galaxies at by the shape of their spectral energy distributions (SEDs). We identify a population of galaxies with excess emission in the K _s -band, which corresponds to [O iii]+Hβ emission at . This population includes 78% of the bluest galaxies with UV slopes steeper than . We de-redshift and scale this photometry to build two composite SEDs, enabling us to measure equivalent widths of these extreme [O iii]+Hβ emission line galaxies (EELGs) at . We identify 60 galaxies that comprise a composite SED with a [O iii]+Hβ rest-frame equivalent width of 803 ± 228 Å and another 218 galaxies in a composite SED with an equivalent width of 230 ± 90 Å. These EELGs are analogous to the “green peas” found in the SDSS and are thought to be undergoing their first burst of star formation due to their blue colors ( ), young ages ( ), and low dust attenuation values. Their strong nebular emission lines and compact sizes (typically ∼1.4 kpc) are consistent with the properties of the star-forming galaxies possibly responsible for reionizing the universe at . Many of the EELGs also exhibit Lyα emission. Additionally, we find that many of these sources are clustered in an overdensity in the Chandra Deep Field South, with five spectroscopically confirmed members at . The spatial distribution and photometric redshifts of the zfourge population further confirm the overdensity highlighted by the EELGs.