A Study of Lyman-alpha Quasar Absorbers in the Nearby Universe

Spectroscopy of ten quasars obtained with the Goddard High Resolution Spectrograph (GHRS) of the Hubble Space Telescope (HST) is presented. We detect 357 absorption lines above a significance level of 3σ in the ten sightlines, and 272 lines above a significance level of 4.5σ. Automated software is used to detect and identify the lines, almost all of which are unresolved at the GHRS G140L resolution of 200 kms. After identifying galactic lines, intervening metal lines, and higher order Lyman lines, we are left with 139 Lyα absorbers in the redshift range 0 < z < 0.22 (lines within 900 km s of geocoronal Lyα are not selected). These diffuse hydrogen absorbers have column densities that are mostly in the range 10 to 10 cm for an assumed Doppler parameter of 30 kms. The number density of lines above a rest equivalent width of 0.24 Å, dN/dz = 38.3± 5.3, agrees well with the the measurement from the Quasar Absorption Line Key Project. There is marginal evidence for cosmic variance in the number of absorbers detected among the ten sightlines. A clustering analysis reveals an excess of nearest neighbor line pairs on velocity scales of 250-750 km s at a 95-98% confidence level. The hypothesis that the absorbers are randomly distributed in velocity space can be ruled out at the 99.8% confidence level. No two-point correlation power is detected (ξ < 1 with 95% confidence). Lyα absorbers have correlation amplitudes on scales of 250-500 kms at least 4-5 times smaller than the correlation amplitude of bright galaxies. A detailed comparison between absorbers in nearby galaxies is carried out on a limited subset of 11 Lyα absorbers where the galaxy sample in a large contiguous volume is complete to MB = −16. Absorbers lie preferentially in regions of intermediate galaxy density but it is often not possible to uniquely assign a galaxy counterpart to an absorber. This sample provides no explicit support for the hypothesis that absorbers are preferentially associated with the halos of luminous galaxies. We have made a preliminary comparison of the absorption line properties and environments with the results of hydrodynamic simulations. The results suggest that the Lyα absorbers represent diffuse or shocked gas in the IGM that traces the cosmic web of large scale structure. Subject headings: galaxies: halos – intergalactic medium – large scale structure of the universe – quasars: absorption lines