Statistics Martin A. Zwaan , Marc A. W. Verheijen , Frank H. Briggs, PASA, 16 (1), in press.
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Conclusions and Discussion
We have used the present knowledge of the nearby galaxy population to estimate the Há I column density distribution function at z=0. It is shown that
undergoes strong redshift evolution from 
to the present, especially at the high column densities. The observed evolution in critically depends on whether the census of Há I in the local Universe is complete. Surveys in Há I and the optical indicate that the density of visible light and neutral gas is dominated by luminous, high surface brightness galaxies. The Há I surveys routinely reach column density limits much lower than what is required to detect the z=0 counterparts of DL
systems. Since Há I mass functions published to date typically lose sensitivity below
, the region of parameter space still open to hide a large amount of high column density gas is that of low Há I masses. Observations to measure the space density of these small Há I masses (Há I clouds and extreme LSB dwarf galaxies) and to evaluate to what extent they contribute to the Há I density and the CDDF of the local Universe are important next steps.
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