The Neutral Hydrogen Content of Galaxies in Cosmological Hydrodynamic Simulations

We examine the global HI properties of galaxies in quarter-billion particle cosmological hydrodynamic simulations, focusing on how our main adjustable physical process, galactic outflows, impacts HI content. In addition to the three outflow models considered in our earlier papers, we present a new one (ezw) motivated by high resolution interstellar medium simulations, in which the scalings of wind speeds and mass loading factors follow those 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, as well as an observationally-constrained transition from neutral to molecular hydrogen. We find that our ezw model produces an HI mass function whose shape agrees well with observations from the ALFALFA survey, having a low mass end slope of -1.3, while other models agree less well. Outflows critically govern the HI content in low-mass galaxies, with higher mass loading factors yielding higher HI fractions. Satellite galaxies have a bimodal distribution in HI fraction versus halo mass, with lower mass satellites and/or satellites in larger halos more often being devoid of HI. 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. At higher redshifts, massive HI galaxies become less frequent and the HI mass function becomes significantly steeper. The global cosmic HI density conspires to remain fairly constant from z~5-0, but the relative contribution from smaller galaxies increases substantially with redshift. Overall, HI in galaxies reflects a transient reservoir of fuel for star formation, and hence provides a crucial glimpse into the inflow and outflow processes that govern galaxy evolution.