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 photo-ionizing background at high redshifts - on
the neutral atomic hydrogen (HI) mass function, the $b_{\rm J}$ 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 SNe 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 photo-ionization 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
SNe 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.
