THE H-I MASS FUNCTION OF GALAXIES FROM A DEEP SURVEY IN THE 21 CENTIMETER LINE

The H I mass function (HIMF) for galaxies in the local universe is con structed from the results of the Arecibo H I Strip Survey, a blind ext ragalactic survey in the 21 cm line. The survey, consisting of two str ips covering in total similar to 65 deg(2) of sky, with a depth of cz = 7400 km s(-1) was optimized to detect column densities of neutral ga s N-HI > 10(18) cm(-2) (5 sigma). The survey yielded 66 significant ex tragalactic signals, of which approximately 50% are cataloged galaxies . No free-floating H I clouds without stars are found. VLA follow-up o bservations of all signals have been used to obtain better measurement s of the positions and fluxes and to allow an alternate determination of the achieved survey sensitivity. The resulting HIMF has a shallow f aint-end slope (alpha approximate to 1.2) and is consistent-with earli er estimates computed for the population of optically selected gas-ric h galaxies. This implies that there is not a large population of gas-r ich low-luminosity or low-surface brightness galaxies that has gone un noticed by optical surveys. The influence of large-scale structure on the determination of the HIMF from the Arecibo H I Strip Survey was te sted by numerical experiments and was not found to affect the resultin g HIMF significantly. The cosmological mass density of H I at the pres ent time, determined from the survey, Omega(HI)(z = 0) = (2.0 +/- 0.5) x 10(-4) h(-1), is in good agreement with earlier estimates. We deter mine lower limits for the average column densities (N-HI) of the galax ies detected in the survey and find that none of the galaxies have (N- HI) < 10(19.7) cm(-2), although there are no observational selection c riteria against finding lower density systems. Eight percent of the si gnals detected in the original survey originated in groups of galaxies whose signals chanced to coincide in frequency.