The impact of environment and mergers on the HI content of galaxies in
  hydrodynamic simulations
  We quantitatively examine the effects of accretion and environment on the HI
content of galaxies within a cosmological hydrodynamic simulation that
reproduces basic observed trends of HI in galaxies. We show that our model
broadly reproduces the observed scatter in HI at a given stellar mass as
quantified by the HI mass function in bins of stellar mass, as well as the HI
richness versus local galaxy density. This shows that the predicted HI
fluctuations and environmental effects are roughly consistent with data with
few minor discrepancies. For satellite galaxies in >= 10^12M_* halos, the HI
richness distribution is bimodal and drops towards the largest halo masses. The
depletion rate of HI once a galaxy enters a more massive halo is more rapid at
higher halo mass, in contrast to the specific star formation rate which shows
much less variation in the attenuation rate versus halo mass. This suggests
that, up to halo mass scales probed here (<= 10^14M_*), star formation is
mainly attenuated by starvation, but HI is additionally removed by stripping
once a hot gaseous halo is present. In low mass halos, the HI richness of
satellites is independent of radius, while in high mass halos they become
gas-poor towards the center. We show that the gas fraction of satellite and
central galaxies decreases from z=5 to z=0,tracking each other until z~1 after
which the satellites' HI content drops much more quickly, particularly for the
highest halo masses. Mergers somewhat increase the HI richness and its scatter
about the mean relation, tracking the metallicity in a way consistent with
arising from inflow fluctuations, while star formation is significantly boosted
relative to HI.
