2017ApJ...847..134K

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, log(M/M_☉) 9 (9.5) at z = 1.3 (2.0). This method, when applied to a mock catalog with the photometric-redshift precision (σ_z/(1+z) 0.02) of ZFOURGE, accurately recovers galaxies in low- and high-density environments. We quantify the environmental quenching efficiency and show that at z 0.5, 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 (log(M/M_☉) 10.5) at all redshifts. For lower-mass galaxies (log(M/M)_☉) 10), the environmental quenching efficiency is very low at z 1.5, but increases rapidly with decreasing redshift. Environmental quenching can account for nearly all quiescent lower-mass galaxies (log(M/M_☉)∼9–10), which appear primarily at z 1.0. 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.