The H i detection of low column density clouds and galaxies

Abstract

The HIDEEP survey was carried out in an attempt to find objects having low inferred neutral hydrogen column densities, yet it found a distribution that was strongly peaked at 1020.65 cm−2. In an attempt to understand this distribution and similar survey results, we model H i profiles of gas discs and use simple simulations of objects having a wide range of H i properties in the presence of an ionizing background. We find that inferred column density (N0Hi) values, which are found by averaging total H i masses over some disc area, do not vary strongly with central column density (Nmax) for detectable objects, so that even a population having a wide range of Nmax values will give rise to a strongly peaked distribution of N0Hi values. We find that populations of objects having a wide range of model parameters give rise to inferred column density distributions around 1020.6±0.3 cm−2. However, populations of fairly massive objects having a wide range of central column densities work best in reproducing the HIDEEP data, and these populations are also consistent with observed Lyman limit absorber counts. It may be necessary to look two orders of magnitude fainter than HIDEEP limits to detect ionized objects having central column densities <1020 cm−2, but the inferred column densities of already detected objects might be lower if their radii could be estimated more accurately.