Lyα Absorption around Nearby Galaxies

We have used STIS aboard HST to search for Lyα absorption lines in the outer regions of eight nearby galaxies using background QSOs and AGN as probes. Lyα lines are detected within a few hundred km s−1 of the systemic velocity of the galaxy in all cases. We conclude that a background line of sight which passes within 26 − 200 h−1 kpc of a foreground galaxy is likely to intercept low column density neutral hydrogen with logN(H I)∼ 13.0. The ubiquity of detections implies a covering factor of ≃ 100 % for low N(H I) gas around galaxies within 200 h−1 kpc. We find, however, that the Lyα lines are usually composed of individual components spread out in velocity over ranges of 300−900 km s−1. Two sightlines show components which are unusually broad for low-redshift Lyα systems, with Doppler parameters ∼ 150 km s−1. These may arise in intragroup gas at temperatures of 1 − 2×106 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 anti-correlation of Lyα equivalent width and impact parameter found at higher redshift. The anti-correlation 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 Lyα complexes (when individual components are summed over ∼ 1000 km s−1) correlate well with a simple estimate of the volume density of galaxies brighter than MB = −17.5 at the same redshift as a Lyα complex. We do not reject the hypothesis that the selected galaxies are directly responsible for the observed Lyα lines, but our analysis indicates that absorption by clumpy intragroup gas is an equally likely explanation. Subject headings: quasars:absorption lines—galaxies:halos — intergalactic medium — large-scale structure of uni-