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 \xi _ { ga } ( v, \rho ) > 1 and with galaxy impact parameter \rho < 200 h ^ { -1 } kpc , we confirm and improve the results presented by Lanzetta et al .
( 1995 ) and Chen et al .
( 1998 ) 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 ( \mbox { H I } ) \gtrsim 10 ^ { 14 } { cm ^ { -2 } } ) around galaxies scales with galaxy B -band luminosity as r \propto L _ { B } ^ { 0.39 \pm 0.09 } , 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 \propto L _ { K } ^ { 0.28 \pm 0.08 } , and ( 5 ) tenuous gas around typical L _ { * } galaxies is likely to be distributed in spherical halos of radius \approx 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 { Ly } \alpha 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 \rho = 12.4 to 175.2 h ^ { -1 } kpc with a median of \rho = 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 { Ly } \alpha absorbing gas around galaxies and galaxy B -band luminosity , the results of our analysis also suggest that the number density evolution of { Ly } \alpha absorption systems may serve to constrain the evolution of the comoving galaxy B -band luminosity density ( at least for the redshift interval between z \sim 0 and z \sim 1 that has been studied in our survey ) .