Previous
abstract 
Next
abstract
The evolution of the neutral hydrogen content of galaxies as a function of time is an important constraint on processes in galactic evolution. We present a comprehensive, statistical description of the H\,I content and distribution within galaxies at the present epoch and compare these statistics with the properties of H\,I associated with ``damped Lyman $\alpha$'' absorption systems at high redshift that are observed in the spectra of QSOs. $\Omega_{HI}(z\!=\!0)$, the H\,I mass density at the present epoch relative to the present critical mass density, is found to be $(2.3 \pm 0.6) \times 10^{-4} h_{75}^{-1}$, consistent with the decreasing trend of the H\,I content with time deduced from QSO absorption line statistics for redshifts from about 4 to 0.5 (Lanzetta 1993). Spiral galaxies contain an overwhelming 94\% of this neutral hydrogen mass. The rest is contained in irregulars (3\%), and S0s plus ellipticals (3\%). Spirals also offer the largest cross-section to line-of-sight absorption of light from QSOs. By considering nearby spirals as potential absorbers, the interception probability as a function of the H\,I column density, N(H\,I), is derived for comparison with the cross-sections inferred from observations of damped Lyman $\alpha$ systems. Consistent with previous studies, the comparison shows that the damped Ly $\alpha$ lines are created by absorbers that subtend larger cross-sections than present-day spirals by a factor of 5 implying that galaxies were either larger or more numerous at $z \sim 2.5$.
We are also investigating the statistics of damped Lyman $\alpha$ absorbers
in the redshift range $0.2
