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

The instantaneous H I 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 H I in galaxies. We show that this model reproduces the observed H I mass function in bins of stellar mass, as well as the H I richness (M_{H I}/M_*) versus local galaxy density. For satellite galaxies in massive ( ≳ 10¹² M_⊙) haloes, the H I richness distribution is bimodal and the median drops towards the largest halo masses. The depletion time-scale of H I 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 ( ≲ 10¹⁴ M_⊙), star formation is mainly attenuated by starvation, but H I is additionally removed by stripping once a hot gaseous halo is present. In low-mass haloes, the H I 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 H I 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 H I.