Reconciling Damped Ly-alpha Statistics and 21cm Studies at z=0

Abstract

Blind 21cm surveys in the local universe have shown that the local HI mass density, Omega_HI, is dominated by luminous, high surface brightness, spiral galaxies. On the other hand, surveys for host galaxies of damped Ly-alpha systems have not always been successful in finding bright spiral galaxies. From an analysis of 21cm aperture synthesis maps of nearby galaxies we show that this apparent contradiction can be resolved by realizing that the HI mass density is dominated by L* galaxies, but the HI cross section near the DLA threshold is more evenly distributed over galaxies with a large range in luminosity, gas mass, and surface brightness. The distributions of column densities and impact parameters of optically identified and non-identified DLA host galaxies in the literature and the HI maps are qualitatively in agreement. Due to poor number statistics of low redshift DLA systems, there is no firm indication that the redshift number count of low redshift DLA systems is inconsistent with that calculated from the nearby galaxy population.