On the role of feedback in shaping the cosmic abundance and clustering of neutral atomic hydrogen in galaxies

We investigate the impact of feedback - from supernovae (SNe), active galactic nuclei (AGN) and a photoionizing background at high redshifts - on the neutral atomic hydrogen (H I) mass function, the b(J)-band luminosity function, and the spatial clustering of these galaxies at z = 0. We use a version of the semi-analytical galaxy formation model GALFORM that calculates self-consistently the amount of HI in a galaxy as a function of cosmic time and links its star formation rate to its mass of molecular hydrogen (H-2). We find that a systematic increase or decrease in the strength of SN feedback leads to a systematic decrease or increase in the amplitudes of the luminosity and HI mass functions, but has little influence on their overall shapes. Varying the strength of AGN feedback influences only the numbers of the brightest or most HI massive galaxies, while the impact of varying the strength of photoionization feedback is restricted to changing the numbers of the faintest or least HI massive galaxies. Our results suggest that the HI mass function is a more sensitive probe of the consequences of cosmological reionization for galaxy formation than the luminosity function. We find that increasing the strength of any of the modes of feedback acts to weaken the clustering strength of galaxies, regardless of their HI richness. In contrast, weaker AGN feedback has little effect on the clustering strength, whereas weaker SN feedback increases the clustering strength of HI-poor galaxies more strongly than HI-rich galaxies. These results indicate that forthcoming HI surveys on next-generation radio telescopes such as the Square Kilometre Array and its pathfinders will be exploited most fruitfully as part of multiwavelength survey campaigns.