Formation and evolution of low HI mass galaxies are affected by environment and by the photoionisation feedback from the UV background after end of reionization. We study the physical processes of low HI mass galaxy formation which are imprinted on the distribution of neutral hydrogen in the Universe using the hierarchical galaxy formation model, GALFORM.
We calculate the sensitivity of correlation functions to the HI mass threshold at redshifts $0 \le z \le 0.5$. Parameterizing clustering as $\xi(z)=(r/r_{0})^{-\gamma}$, we find that low HI mass galaxies increase the clustering amplitude $r_{0}$ and slope $\gamma$ in HI selected galaxy samples. This is opposite to expectations from optical galaxy surveys. We show the HI mass function for different host dark matter halo masses and types (central or satellite) to interpret the clustering of HI galaxies.
We also show the contribution of low HI mass galaxies to 21cm intensity mapping. We find that semi-analytic modelling of intensity mapping requires a dark matter halo mass resolution of $<$10$^{10}{\rm h}^{-1}$M$_{\odot}$ in order to correctly predict 21cm brightness temperature fluctuations.