Evidence for structure in the H I column density distribution of QSO absorbers

The H I column density distribution function of QSO absorption line systems is investigated using recent data with high spectral resolution, and extensive surveys of the Lyman limit systems and damped Ly α systems. The hypothesis that the differential distribution function is fitted by a single power law is rejected at the 99 percent confidence level. A double power law, with a break at N(H I) = 1016 cm-2, also provides a poor fit over the range in which the sample is complete. While there are no discontinuities in the observed distribution, there is a clear flattening at N(H I) ≈ 1016 cm-2, compared to lower column densities. These observed features can be understood using models of photoionized clouds which are confined by an external pressure with density profiles governed by gravity. In particular, the flattening at N(H I) ~ 1016 cm-2 can be explained in terms of a transition between metal-poor and metal-rich systems.