Conditional quenching: A detailed look at the SFR'density relation at z 1/4 0.9 from ORELSE

Abstract

We present a study of the star formation rate (SFR)-density relation at z 1/4 0.9 using data drawn from the Observations of Redshift Evolution in Large Scale Environments (ORELSE) survey. We find that SFR does depend on environment, but only for intermediate-stellar mass galaxies (10 10.1 < 10 10.8) wherein the median SFR at the highest densities is 0.2-0.3 dex less than at lower densities at a significance of 4σ. Galaxies that are more/less massive than this have SFRs that vary at most by approx 20 rm per cent across all environments, but show no statistically significant trend. We further split galaxies into low-redshift (z 1/4 0.8) and high-redshift (z 1/4 1.05) subsamples and observe nearly identical behaviour. We devise a simple toy model to explore possible star formation histories for galaxies evolving between these redshifts. The key assumption in this model is that star-forming galaxies in a given environment-stellar mass bin can be described as a superposition of two exponential time-scales (SFR ' e 't/'.,): a long''., time-scale with '., = 4 Gyr to simulate 'normal' star-forming galaxies, and a short-'., time-scale with free '., (between 0.3 ≤ '.,/Gyr ≤ 2) to simulate galaxies on a quenching trajectory. In general, we find that galaxies residing in low/high environmental densities are more heavily weighted to the long-'.,/short-'., pathways, respectively, which we argue is a signature of environmental quenching. Furthermore, for intermediate-stellar mass galaxies this transition begins at intermediate-density environments suggesting that environmental quenching is relevant in group-like haloes and/or cluster infall regions.