2001ApJ...559..654C

We present results of a program to obtain and analyze HST WFPC2 images and ground-based images of galaxies identified in an imaging and spectroscopic survey of faint galaxies in fields of HST spectroscopic target QSOs. Considering a sample of physically correlated galaxy and absorber pairs with galaxy-absorber cross-correlation amplitude ξ_ga(v,ρ)>1 and with galaxy impact parameter ρ<200 h^–1 kpc, we confirm and improve the results presented by Lanzetta et al. and Chen et al. that (1) extended gaseous envelopes are a common and generic feature of galaxies of a wide range of luminosity and morphological type, (2) the extent of tenuous gas [N(H I)≳10¹⁴ cm^–2] around galaxies scales with galaxy B-band luminosity as r∝L^0.39±0.09_B, and (3) galaxy interactions do not play an important role in distributing tenuous gas around galaxies in most cases. We further demonstrate that (4) the gaseous extent of galaxies scales with galaxy K-band luminosity as r∝L^0.28±0.08_K, and (5) tenuous gas around typical L_* galaxies is likely to be distributed in spherical halos of radius ~180 h^–1 kpc of covering factor of nearly unity. The sample consists of 34 galaxy and absorber pairs and 13 galaxies that do not produce Lyα absorption lines to within sensitive upper limits. Redshifts of the galaxy and absorber pairs range from z=0.0752 to 0.8920 with a median of z=0.3567; impact parameter separations of the galaxy and absorber pairs range from ρ=12.4 to 175.2 h^–1 kpc with a median of ρ=62.2 h^–1 kpc. Of the galaxies, 15 (32%) are of B-band luminosity L_B<0.25 L_B* and six (13%) are of low surface brightness. The galaxy sample is therefore representative of the galaxy population over a large fraction of the Hubble time. Because galaxies of all morphological types possess extended gaseous halos and because the extent of tenuous gas around galaxies scales with galaxy K-band luminosity, we argue that galaxy mass–rather than recent star formation activity–is likely to be the dominant factor that determines the extent of tenuous gas around galaxies. Nevertheless, applying the scaling relationship between the extent of Lyα-absorbing gas around galaxies and galaxy B-band luminosity, the results of our analysis also suggest that the number density evolution of Lyα absorption systems may serve to constrain the evolution of the comoving galaxy B-band luminosity density (at least for the redshift interval between z∼0 and z∼1 that has been studied in our survey).