Density Distribution Function at z = 0 from H

We have measured the column density distribution function, f(NHI), at z = 0 using 21-cm Hi emission from galaxies selected from a blind Hi survey. f(NHI) is found to be smaller and flatter at z = 0 than indicated by high-redshift measurements of Damped Lyman-α (DLA) systems, consistent with the predictions of hierarchical galaxy formation. The derived DLA number density per unit redshift, dNDLA/dz =0.058, is in moderate agreement with values calculated from low-redshift QSO absorption line studies. We use two different methods to determine the types of galaxies which contribute most to the DLA cross-section: comparing the power law slope of f(NHI) to theoretical predictions and analysing contributions to dNDLA/dz. We find that comparison of the power law slope cannot rule out spiral discs as the dominant galaxy type responsible for DLA systems. Analysis of dNDLA/dz however, is much more discriminating. We find that galaxies with logMHI < 9.0 make up 34% of dNDLA/dz; Irregular and Magellanic types contribute 25%; galaxies with surface brightness, μ25> 24 mag arcsec account for 22% and sub-L∗ galaxies contribute 45% to dNDLA/dz. We conclude that a large range of galaxy types give rise to DLA systems, not just large spiral galaxies as previously speculated.