On the Origin of the Hubble Sequence: I. Insights on Galaxy Color
  Migration from Cosmological Simulations
  An analysis of more than 3000 galaxies resolved at better than 114 pc/h at
z=0.62 in a LAOZI cosmological adaptive mesh refinement hydrodynamic simulation
is performed and insights gained on star formation quenching and color
migration. The vast majority of red galaxies are found to be within three
virial radii of a larger galaxy, at the onset of quenching when the specific
star formation rate experiences the sharpest decline to fall below
~10^{-2}-10^{-1}/Gyr (depending on the redshift). We shall thus call this
mechanism "environment quenching", which encompasses satellite quenching. Two
physical processes are largely responsible: ram-pressure stripping first
disconnects the galaxy from the cold gas supply on large scales, followed by a
longer period of cold gas starvation taking place in high velocity dispersion
environment, during the early part of which the existing dense cold gas in the
central region (<10kpc) is consumed by in situ star formation. Quenching is
found to be more efficient, but not faster, on average, in denser environment.
Throughout this quenching period and the ensuing one in the red sequence
galaxies follow nearly vertical tracks in the color-stellar-mass diagram. In
contrast, individual galaxies of all masses grow most of their stellar masses
in the blue cloud, prior to the onset of quenching, and progressively more
massive blue galaxies with already relatively older mean stellar ages continue
to enter the red sequence. Consequently, correlations among observables of red
galaxies - such as the age-mass relation - are largely inherited from their
blue progenitors at the onset of quenching. While the color makeup of the
entire galaxy population strongly depends on environment, which is a direct
result of environment quenching, physical properties of blue galaxies as a
sub-population show little dependence on environment.
