The spatial distribution of neutral hydrogen as traced by low HI mass
  galaxies
  The formation and evolution of galaxies with low neutral atomic hydrogen (HI)
masses, M$_{\rm HI}$$<$10$^{8}h^{-2}$M$_{\odot}$, are affected by host dark
matter halo mass and photoionisation feedback from the UV background after the
end of reionization. We study how the physical processes governing the
formation of galaxies with low HI mass are imprinted on the distribution of
neutral hydrogen in the Universe using the hierarchical galaxy formation model,
GALFORM. We calculate the effect on the correlation function of changing the HI
mass detection threshold at redshifts $0 \le z \le 0.5$. We parameterize the
clustering as $\xi(r)=(r/r_{0})^{-\gamma}$ and we find that including galaxies
with M$_{\rm HI}$$<$10$^{8}h^{-2}$M$_{\odot}$ increases the clustering
amplitude $r_{0}$ and slope $\gamma$ compared to samples of higher HI masses.
This is due to these galaxies with low HI masses typically being hosted by
haloes with masses greater than 10$^{12}{h}^{-1}$M$_{\odot}$, and is in
contrast to optically selected surveys for which the inclusion of faint, blue
galaxies lowers the clustering amplitude. We show the HI mass function for
different host dark matter halo masses and galaxy types (central or satellite)
to interpret the values of $r_{0}$ and $\gamma$ of the clustering of
HI-selected galaxies. We also predict the contribution of low HI mass galaxies
to the 21cm intensity mapping signal. We calculate that a dark matter halo mass
resolution better than $\sim$10$^{10}{h}^{-1}$M$_{\odot}$ at redshifts higher
than 0.5 is required in order to predict converged 21cm brightness temperature
fluctuations.
