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 ~
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* / Msol < 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. Interestingly, mass does not
drive SFR; galaxies that are more/less massive have SFRs that vary at most by
~20% across all environments showing no statistically significant dependence.
We further split galaxies into low-redshift (z ~ 0.8) and high-redshift (z ~
1.05) subsamples and observe nearly identical behavior. We devise a simple toy
model to explore possible star-formation histories (SFHs) 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 timescales (SFR ~ e^(-t/tau)): a long-tau
timescale with tau = 4 Gyr to simulate "normal" star-forming galaxies, and a
short-tau timescale 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 halos and/or cluster infall
regions.
