ESTIMATING THE MASS DENSITY OF NEUTRAL GAS AT Z-LESS-THAN-1

We use the relationships between galactic H I mass and B-band luminosi ty determined by Rao & Briggs to recalculate the mass density of neutr al gas at the present epoch based on more recent measures of the galax y luminosity function than were available to those authors. We find Om ega(gas)(z=0)similar or equal to 5 x 10(-4) in good agreement with the original Rao & Briggs value, suggesting that this quantity is now rea sonably secure. We than show that, if the scaling between H I mass and B-band luminosity has remained approximately consistent since z=1, th e evolution of the luminosity function found by the Canada-France Reds hift Survey translates to an increase of Omega(gas) by a factor of app roximate to 3 at z=0.5-1. A similar value is obtained quite independen tly from consideration of the luminosity function of Mg II absorbers a t z=0.65. By combining these new estimates with data from damped Lyman alpha systems at higher redshift, it is possible to assemble a rough sketch of the evolution of Omega(gas) over the last 90 per cent of the age of the Universe. The consumption of H I gas with time is in broad agreement with models of chemical evolution which include the effects of dust, although more extensive samples of damped Lyman alpha system s at low and intermediate redshift are required for a quantitative ass essment of the dust bias.