The Distribution of Neutral Hydrogen in the Local Universe

Hydrogen is the most abundant element in the Universe. It can be characterized as being in the ionized, neutral atomic (HI), or molecular (H_2) states. Though hydrogen gas is almost completely ionized in the post- re-ionization era, there are still traces of HI locked up inside galaxies, providing fuel for future star formation of the galaxy. HI is therefore a useful probe to study the formation and distribution of large-scale structures in the Universe. I will discuss how different populations of galaxies contribute to the total HI budget, Omega_HI, in the local Universe. I will present how these populations of galaxies make up the total HI mass function, the HI width function, and the HI velocity function. Also, I will describe a model, based on these results, to populate HI in dark matter halos motivated and calibrated from recent observations.