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-α absorption systems at high redshift that are observed in the spectra of QSOs. Ω HI (z=0), the H I mass density at the present epoch relative to the present critical mass density, is found to be (2.5±0.6)×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)
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