The impact of environment and mergers on the HI content of galaxies in hydrodynamic simulations

The instantaneous HI content of galaxies is thought to be governed by recent accretion and environment. We examine these effects within a cosmological hydrodynamic simulation that includes a heuristic galactic outflow model that reproduces basic observed trends of HI in galaxies. We show that this model reproduces the observed HI mass function in bins of stellar mass, as well as the HI richness (M-HI / M-*) versus local galaxy density. For satellite galaxies in massive (greater than or similar to 10(12)M(circle dot)) haloes, the HI richness distribution is bimodal and the median drops towards the largest halo masses. The depletion time-scale of HI entering a massive halo is more rapid, in contrast to the specific star formation rate which shows little variation in the attenuation rate versus halo mass. This suggests that, up to the halo mass scales probed here (less than or similar to 10(14)M(circle dot)), star formation is mainly attenuated by starvation, but HI is additionally removed by stripping once a hot gaseous halo is present. In low-mass haloes, the HI richness of satellites is independent of radius, while in very massive haloes they become gas-poor towards the centre, confirming the increasing strength of the stripping with halo mass. Mergers somewhat increase the HI richness and its scatter about the mean relation, tracking the metallicity in a way consistent with it arising from inflow fluctuations, while star formation is significantly boosted relative to HI.