Evolution of star formation rate-density relation over cosmic time in a
  simulated universe: the observed reversal reproduced
  We use the IllustrisTNG cosmological hydrodynamical simulation to study the
evolution of star formation rate (SFR)-density relation over cosmic time. We
construct several samples of galaxies at different redshifts from $z=2.0$ to
$z=0.0$, which have the same comoving number density. The SFR of galaxies
decreases with local density at $z=0.0$, but its dependence on local density
becomes weaker with redshift. At $z\gtrsim1.0$, the SFR of galaxies increases
with local density (reversal of the SFR-density relation), and its dependence
becomes stronger with redshift. This change of SFR-density relation with
redshift still remains even when fixing the stellar masses of galaxies. The
dependence of SFR on the distance to a galaxy cluster also shows a change with
redshift in a way similar to the case based on local density, but the reversal
happens at a higher redshift, $z\sim1.5$, in clusters. On the other hand, the
molecular gas fraction always decreases with local density regardless of
redshift at $z=0.0-2.0$ even though the dependence becomes weaker when we fix
the stellar mass. Our study demonstrates that the observed reversal of the
SFR-density relation at $z\gtrsim1.0$ can be successfully reproduced in
cosmological simulations. Our results are consistent with the idea that
massive, star-forming galaxies are strongly clustered at high redshifts,
forming larger structures. These galaxies then consume their gas faster than
those in low-density regions through frequent interactions with other galaxies,
ending up being quiescent in the local universe.
