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We examine the global HI properties of galaxies in quarter-billion particlecosmological simulations using Gadget-2, focusing on how galactic outflowsimpact HI content. We consider four outflow models, including a new one (ezw)motivated by recent interstellar medium simulations in which the wind speed andmass loading factor scale as expected for momentum-driven outflows for largergalaxies and energy-driven outflows for dwarfs (sigma<75 km/s). To obtainpredicted HI masses, we employ a simple but effective local correction forparticle self-shielding, and an observationally-constrained transition fromneutral to molecular hydrogen. Our ezw simulation produces an HI mass functionwhose faint-end slope of -1.3 agrees well with observations from the ALFALFAsurvey; other models agree less well. Satellite galaxies have a bimodaldistribution in HI fraction versus halo mass, with smaller satellites and/orthose in larger halos more often being HI-deficient. At a given stellar mass,HI content correlates with star formation rate and inversely correlates withmetallicity, as expected if driven by stochasticity in the accretion rate. Tohigher 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.

The Neutral Hydrogen Content of Galaxies in Cosmological Hydrodynamic Simulations