AN HI SELECTED SAMPLE OF GALAXIES - THE HI MASS FUNCTION AND THE SURFACE BRIGHTNESS DISTRIBUTION

Results from the Arecibo HI Strip Survey, an unbiased extragalactic HI survey, combined with optical and 21 cm follow-up observations, deter mine the HI mass function and the cosmological mass density of HI at t he present epoch. Both are consistent with earlier estimates, computed for the population of optically selected galaxies. This consistency o ccurs because, although the distribution of optical central surface br ightnesses among galaxies is flat, we fail to find a population of gal axies with central surface brightnesses fainter than 24 B-mag arcsec(- 2), even though there is no observational selection against them.