The gaseous extent of galaxies and the origin of Lyman-alpha absorption systems: A survey of galaxies in the fields of Hubble Space Telescope spectroscopic target QSOs

We present initial results of an imaging and spectroscopic survey of faint galaxies in fields of Hubble Space Telescope spectroscopic target QSOs. The primary objectives of the survey are (1) to determine the incidence, extent, and covering factor of extended gaseous envelopes of luminous galaxies and (2) to determine the fraction of Lyα absorption systems that arise in luminous galaxies. The goal of the survey is to identify in each field under consideration all objects with apparent r-band magnitudes satisfying r 160 h^-1^ kpc gives rise to Lyα absorption. At least eight of 23 Lyα absorption systems arise in galaxies. On the basis of these results we reach the following conclusions: 1. At z <~ 1 most luminous galaxies are surrounded by extended gaseous envelopes of ~160 h^-1^ kpc radius and of roughly unit covering factor. This conclusion confirms previous speculation that normal, luminous galaxies possess extended gaseous halos or extended gaseous disks. 2. At z <~ 1 the fraction of Lyα absorption systems-including Lyα-forest absorption systems-that arise in luminous galaxies is at least 0. 32 +/- 0. 10 and may be as high as 0. 60 +/- 0. 19. This conclusion runs contrary to the longstanding belief that Lyα-forest absorption systems arise in intergalactic clouds.