Evolved Galaxies in High-Density Environments Across 2.0≤ Z<4.2 Using the ZFOURGE Survey

To explore the role environment plays in influencing galaxy evolution at high redshifts, we study 2.0≤ z<4.2 environments using the FourStar Galaxy Evolution (ZFOURGE) survey. Using galaxies from the COSMOS legacy field with log(M_*/M_⊙)≥9.5, we use a seventh nearest neighbour density estimator to quantify galaxy environment, dividing this into bins of low, intermediate and high density. We discover new high density environment candidates across 2.0≤ z<2.4 and 3.1≤ z<4.2. We analyse the quiescent fraction, stellar mass and specific star formation rate (sSFR) of our galaxies to understand how these vary with redshift and environment. Our results reveal that, across 2.0≤ z<2.4, the high density environments are the most significant regions, which consist of elevated quiescent fractions, log(M_*/M_⊙)≥10.2 massive galaxies and suppressed star formation activity. At 3.1≤ z<4.2, we find that high density regions consist of elevated stellar masses but require more complete samples of quiescent and sSFR data to study the effects of environment in more detail at these higher redshifts. Overall, our results suggest that well-evolved, passive galaxies are already in place in high density environments at z∼2.4, and that the Butcher-Oemler effect and SFR-density relation may not reverse towards higher redshifts as previously thought.