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 alpha systems. The hypothesis that the differential distribution function is fitted by a single power law is rejected at the 99 per cent confidence level. A double power law, with a break at N(H I) = 10(16) cm-2, also provides a poor fit over the range in which the sample is complete. Whilst there are no discontinuities in the observed distribution, there is a clear flattening at N(H I) almost-equal-to 10(16) 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) approximately 10(16) cm-2 can be explained in terms of a transition between metal-poor and metal-rich systems.