The Origin of Intermediate-Redshift Lyman Alpha Absorption Systems

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 (a) to determine the incidence, extent, and covering factor of extended gaseous envelopes of luminous galaxies and (b) to determine the fraction of Lyman alpha 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 theta < 1.3 arcmin. We find that at z \apl 1 most luminous galaxies are surrounded by extended gaseous envelopes of approx 160 h^{-1} kpc radius and roughly unit covering factor and that at z \apl 1 the fraction of Ly alpha absorption systems that arise in luminous galaxies is at least 0.35 \pm 0.10 and is likely 0.65 \pm 0.18.