  Modelling the distribution of neutral hydrogen (HI) in dark matter halos is
important for studying galaxy evolution in the cosmological context. We use a
novel approach to infer the HI-dark matter connection at the massive end
($m_{\rm HI} > 10^{9.8} M_{\odot}$) from radio HI emission surveys, using
optical properties of low-redshift galaxies as an intermediary. In particular,
we use a previously calibrated optical HOD describing the luminosity- and
colour-dependent clustering of SDSS galaxies and describe the HI content using
a statistical scaling relation between the optical properties and HI mass. This
allows us to compute the abundance and clustering properties of HI-selected
galaxies and compare with data from the ALFALFA survey. We apply an MCMC-based
statistical analysis to constrain the free parameters related to the scaling
relation. The resulting best-fit scaling relation identifies massive HI
galaxies primarily with optically faint blue centrals, consistent with
expectations from galaxy formation models. We compare the HI-stellar mass
relation predicted by our model with independent observations from matched
HI-optical galaxy samples, finding reasonable agreement. As a further
application, we make some preliminary forecasts for future observations of HI
and optical galaxies in the expected overlap volume of SKA and Euclid/LSST.
