A study of Lyα quasar absorbers in the nearby universe

Spectroscopy of 10 quasars obtained with the Goddard High Resolution Spectrograph (GHRS) of the Hubble Space Telescope (HST) is presented. In the 10 sight lines, we detect 357 absorption lines above a significance level of 3 σ and 272 lines above a significance level of 4.5 σ. Automated software is used to detect and identify the lines, almost all of which are unresolved at the GHRS G140L resolution of 200 km s^-1. 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^-1 of geocoronal Lyα are not selected). These diffuse hydrogen absorbers have column densities that are mostly in the range 10¹³-10¹⁵ cm^-2 for an assumed Doppler parameter of 30 km s^-1. The number density of lines above a rest equivalent width of 0.24 Å, dN/dz = 38.3 ± 5.3, agrees well with the measurement from the Quasar Absorption-Line Key Project. There is marginal evidence for cosmic variance in the number of absorbers detected among the 10 sight lines. A clustering analysis reveals an excess of nearest-neighbor line pairs on velocity scales of 250-750 km s^-1 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 km s^-1, 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 M_B = -16. Absorbers lie preferentially in regions of intermediate galaxy density, but it is often not possible to assign a unique 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 intergalactic medium that traces the cosmic web of large-scale structure.