Estimating the mass density of neutral gas at z < 1

We use the relationships between galactic HI mass and B-band luminositydetermined by Rao & Briggs to recalculate the mass density of neutral gas atthe present epoch based on more recent measures of the galaxy luminosityfunction than were available to those authors. We find $\Omega_{gas}(z=0)\simeq 5 \times 10^{-4}$ in good agreement with the original Rao & Briggsvalue, suggesting that this quantity is now reasonably secure. We then showthat, if the scaling between H I mass and B-band luminosity has remainedapproximately constant since $z = 1$, the evolution of the luminosity functionfound by the Canada-France redshift survey translates to an increase of$\Omega_{gas}$ by a factor of $\approx 3$ at $z = 0.5 - 1$ . A similar value isobtained quite independently from consideration of the luminosity function ofMg II absorbers at $z = 0.65$. By combining these new estimates with data fromdamped \lya systems at higher redshift, it is possible to assemble a roughsketch of the evolution of $\Omega_{gas}$ over the last 90% of the age of theuniverse. The consumption of H I gas with time is in broad agreement withmodels of chemical evolution which include the effects of dust, although moreextensive samples of damped \lya systems at low and intermediate redshift arerequired for a quantitative assessment of the dust bias.Comment: LaTeX file, 11 pages, 1 figure, accepted MNRAS pink page