Lyα absorption around nearby galaxies

We have used the Space Telescope Imaging Spectrograph (STIS) aboard the Hubble Space Telescope to search for Lyalpha absorption lines in the outer regions of eight nearby galaxies using background quasars and AGNs as probes. Lyalpha lines are detected within a few hundred kilometers per second of the systemic velocity of the galaxy in all cases. We conclude that a background line of sight that passes within 26-200 h(-1) kpc of a foreground galaxy is likely to intercept low column density neutral hydrogen with log N(H I)greater than or similar to13.0. The ubiquity of detections implies a covering factor of similar or equal to100% for low N(H I) gas around galaxies within 200 h(-1) kpc. We find, however, that the Lyalpha lines are usually composed of individual components spread out in velocity over ranges of 300-900 km s(-1). Two sight lines show components that are unusually broad for low-redshift Lyalpha systems, with Doppler parameters similar to150 km s(-1). These may arise in intragroup gas at temperatures of (1-2)x10(6) K. We discuss the difficulty in trying to associate individual absorption components with the selected galaxies and their neighbors but show that by degrading our STIS data to lower resolutions, we are able to reproduce the anticorrelation of Lyalpha equivalent width and impact parameter found at higher redshift. The anticorrelation does not improve by correcting for the absolute magnitude of a galaxy in the same way as found at higher z. We also show that the equivalent width and column density of Lyalpha complexes (when individual components are summed over similar to1000 km s(-1)) correlate well with a simple estimate of the volume density of galaxies brighter than M(B) = -17.5 at the same redshift as a Lyalpha complex. We do not reject the hypothesis that the selected galaxies are directly responsible for the observed Lyalpha lines, but our analysis indicates that absorption by clumpy intragroup gas is an equally likely explanation.