Extracting HI Astrophysics from Interferometric Intensity Mapping.

We present a new halo model of neutral hydrogen (HI) calibrated to galaxy formation simulations at redshifts $z\sim0.1$ and $z\sim1.0$ which we employ to investigate the constraining power of interferometric HI Intensity Mapping on HI astrophysics. We demonstrate that constraints on the small-scale HI power spectrum can break the degeneracy between the HI density $\Omega_{\rm HI}$ and the HI bias $b_{\rm HI}$. For $z\sim0.1$, we forecast that an accurate measurement of $\Omega_{\rm HI}$ up to 6% level precision and the large scale HI bias $b_{\rm HI}^0$ up to 1% level precision can be achieved using Square Kilometre Array (SKA) pathfinder data from MeerKAT and ASKAP. We also propose a new description of the HI shot noise in the halo model framework in which the scatter of HI galaxies can be constrained. Furthermore, given the number density of HI galaxies above a certain HI mass threshold, future surveys will also be able to constrain the HI Mass Function using only the HI shot noise. This will lead to constraints at the 10% level using the standard Schechter function. This technique will potentially provide a new way of measuring the HI Mass Function, independent from existing methods. We predict that the SKA will be able to further improve the low redshift constraints by a factor of three, as well as pioneering measurements of HI astrophysics at higher redshifts.