On the Reversal of SFR-Density Relation at z=1: Insights from
  Simulations
  Recent large surveys have found a reversal of the star formation rate
(SFR)-density relation at z=1 from that at z=0 (e.g. Elbaz et al.; Cooper et
al.), while the sign of the slope of the color-density relation remains
unchanged (e.g. Cucciati et al.; Quadri et al.). We use state-of-the-art
adaptive mesh refinement cosmological hydrodynamic simulations of a 21x24x20
(Mpc/h)$^3$ region centered on a cluster to examine the SFR-density and
color-density relations of galaxies at z=0 and z=1. The local environmental
density is defined by the dark matter mass in spheres of radius 1 Mpc/h, and we
probe two decades of environmental densities. Our simulations produce a large
increase of SFR with density at z=1, as in the observations of Elbaz et al. We
also find a significant evolution to z=0, where the SFR-density relation is
much flatter. The color-density relation in our simulations is consistent from
z=1 to z=0, in agreement with observations. We find that the increase in the
median SFR with local density at z=1 is due to a growing population of
star-forming galaxies in higher-density environments. At z=0 and z=1 both the
SFR and cold gas mass are tightly correlated with the galaxy halo mass, and
therefore the correlation between median halo mass and local density is an
important cause of the SFR-density relation at both redshifts. We also show
that the local density on 1 Mpc/h scales affects galaxy SFRs as much as halo
mass at z=0. Finally, we find indications that the role of the 1 Mpc/h scale
environment reverses from z=0 to z=1: at z=0 high-density environments depress
galaxy SFRs, while at z=1 high-density environments tend to increase SFRs.
