The column density distribution function at z = 0 from HI selected galaxies

Abstract

We have measured the column density distribution function, f(N HI), 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 co..