We examine the global HI properties of galaxies in quarter billion particle cosmological
simulations using GADGET-2, focusing on howgalactic outflows impactHI 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 momentumdriven
outflows for larger galaxies and energy-driven outflows for dwarfs (σ <75 km s−1). To
obtain predicted HI masses, we employ a simple but effective local correction for particle selfshielding
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 Arecibo Fast Legacy ALFA 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 haloes more often being HI deficient. At a given stellar mass,
HI content correlates with the 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.