Environmentally Driven Global Evolution of Galaxies
  Utilizing high-resolution large-scale galaxy formation simulations of the
standard cold dark matter model, we examine global trends in the evolution of
galaxies due to gravitational shock heating by collapse of large halos and
large-scale structure. We find two major global trends. (1) The mean specific
star formation rate (sSFR) at a given galaxy mass is a monotonically increasing
function with increasing redshift. (2) The mean sSFR at a given redshift is a
monotonically increasing function of decreasing galaxy mass that steepens with
decreasing redshift. The general dimming trend with time merely reflects the
general decline of gas inflow rate with increasing time. The differential
evolution of galaxies of different masses with redshift is a result of
gravitational shock heating of gas due to formation of large halos (groups and
clusters) and large-scale structure that move a progressively larger fraction
of galaxies and their satellites into environments where gas has too high an
entropy to cool to continue feeding resident galaxies. Overdense regions where
larger halos are preferentially located begin to be heated earlier and have
higher temperatures than lower density regions at any given time, causing sSFR
of larger galaxies to fall below the general dimming trend at higher redshift
than less massive galaxies and galaxies with high sSFR to gradually shift to
lower density environments at lower redshift. We find that several noted cosmic
downsizing phenomena are different manifestations of these general trends. We
also find that the great migration of galaxies from blue cloud to red sequence
as well as color-density relation, among others, may arise naturally in this
picture.
