The Neutral Hydrogen Content of Galaxies in Cosmological Hydrodynamic
  Simulations
  We examine the global HI properties of galaxies in quarter-billion particle
cosmological simulations using Gadget-2, focusing on how galactic outflows
impact HI content. We consider four outflow models, including a new one (ezw)
motivated by recent interstellar medium simulations in which the wind speed and
mass loading factor scale as expected for momentum-driven outflows for larger
galaxies and energy-driven outflows for dwarfs (sigma<75 km/s). To obtain
predicted HI masses, we employ a simple but effective local correction for
particle self-shielding, and an observationally-constrained transition from
neutral to molecular hydrogen. Our ezw simulation produces an HI mass function
whose faint-end slope of -1.3 agrees well with observations from the ALFALFA
survey; other models agree less well. Satellite galaxies have a bimodal
distribution in HI fraction versus halo mass, with smaller satellites and/or
those in larger halos more often being HI-deficient. At a given stellar mass,
HI content correlates with star formation rate and inversely correlates with
metallicity, as expected if driven by stochasticity in the accretion rate. To
higher redshifts, massive HI galaxies disappear and the mass function steepens.
The global cosmic HI density conspires to remain fairly constant from z~5-0,
but the relative contribution from smaller galaxies increases with redshift.
