The gaseous extent of galaxies and the origin of Lyα absorption systems.: III.: Hubble Space Telescope imaging of Lyα-absorbing galaxies at z<1

We present initial results of a program to obtain and analyze Hubble Space Telescope (HST) Wide Field Planetary Camera 2 images of galaxies identified in an imaging and spectroscopic survey of faint galaxies in fields of HST spectroscopic target QSOs. We measure properties of 87 galaxies, of which 33 are associated with corresponding Ly alpha absorption systems and 24 do not produce corresponding Ly alpha absorption lines to within sensitive upper limits. Considering only galaxy and absorber pairs that are likely to be physically associated and excluding galaxy and absorber pairs within 3000 km s(-1) of the background QSOs leaves 26 galaxy and absorber pairs and seven galaxies that do not produce corresponding Ly alpha absorption lines to within sensitive upper limits. Redshifts of the galaxy and absorber pairs range from 0.0750 to 0.8912 with a median of 0.3718, and impact parameter separations of the galaxy and absorber pairs range from 12.4 to 157.4 h(-1) kpc with a median of 62.4 h(-1) kpc. The primary result of the analysis is that the amount of gas encountered along the line of sight depends on the galaxy impact parameter and B-band luminosity but does not depend strongly on the galaxy average surface brightness, disk-to-bulge ratio, or redshift. This result confirms and improves upon the anticorrelation between Ly alpha absorption equivalent width and galaxy impact parameter found previously by Lanzetta et al. in 1995. Spherical halos cannot be distinguished from flattened disks on the basis of the current observations, and there is no evidence that galaxy interactions play an important role in distributing tenuous gas around galaxies in most cases. Galaxies might account for all Ly alpha absorption systems with W > 0.3 Angstrom, but this depends on the unknown luminosity function and gaseous cross sections of low-luminosity galaxies as well as on the uncertainties of the observed number density of Ly alpha absorption systems.