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 < 21.5 within angular distances to the QSOs satisfying θ < 1′.3. The current observations cover six fields and are 37% complete to the goal of the survey. These observations identify 46 galaxies at redshifts spanning z = 0.0700-0.5526 and at impact parameters to the QSOs spanning ρ = 16.6-346.9 h^-1 kpc. Of these galaxies, 11 are coincident in redshift with absorption systems and 21 do not give rise to absorption to within sensitive upper limits. Nine galaxies are coincident in redshift with "Lyα-forest" absorption systems that show Lyα absorption but no corresponding metal-line absorption, and two galaxies are coincident in redshift with C IV absorption systems that show both Lyα and C IV absorption. Various lines of evidence demonstrate that the coincident galaxies are responsible for the corresponding absorption systems and are not present as the result of chance coincidence or merely spatially correlated with the absorption systems. The most important evidence is that there exists a statistical anti-correlation between Lyα rest-frame equivalent width and impact parameter. Each of five galaxies with ρ < 70 h^-1 kpc gives rise to Lyα absorption, whereas only five of 10 galaxies with ρ = 70-160 h^-1 kpc give rise to Lyα absorption, and just one of nine galaxies with ρ > 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.