Conditional quenching: a detailed look at the SFR−density relation at $z$∼ 0.9 from ORELSE

We present a study of the star formation rate (SFR)-density relation at z similar to 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) < M-*/M-circle dot < 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 4 sigma. Galaxies that are more/less massive than this have SFRs that vary at most by approximate to 20 per cent across all environments, but show no statistically significant trend. We further split galaxies into low-redshift (z similar to 0.8) and high-redshift (z similar to 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 proportional to e(-t/tau)): a long-tau time-scale with tau = 4 Gyr to simulate 'normal' star-forming galaxies, and a short-tau time-scale with free tau (between 0.3 <= tau/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-tau/short-tau 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.