Lyα Absorption around Nearby Galaxies

We have used STIS aboard HST to search for Lyman-alpha (Lya) absorption linesin the outer regions of eight nearby galaxies using background QSOs and AGN asprobes. Lya lines are detected within a few hundred km/s of the systemicvelocity of the galaxy in all cases. We conclude that a backgroundline-of-sight which passes within 26-200 h-1 kpc of a foreground galaxy islikely to intercept low column density neutral hydrogen with log N(HI) >~ 13.0.The ubiquity of detections implies a covering factor of ~ 100% for low N(HI)gas around galaxies within 200 h-1 kpc. We discuss the difficulty in trying toassociate individual absorption components with the selected galaxies and theirneighbors, but show that by degrading our STIS data to lower resolutions, weare able to reproduce the anti-correlation of Lya equivalent width and impactparameter found at higher redshift. We also show that the equivalent width andcolumn density of Lya complexes (when individual components are summed over ~1000 km/s) correlate well with a simple estimate of the volume density ofgalaxies brighter than M(B) = -17.5 at the same redshift as a Lya complex. Wedo not reject the hypothesis that the selected galaxies are directlyresponsible for the observed Lya lines, but our analysis indicates thatabsorption by clumpy intragroup gas is an equally likely explanation. (Abriged)