The Local Lyα Forest. III. Relationship between Lyα Absorbers and Galaxies, Voids, and Superclusters

In this paper, we use large-angle, nearby galaxy redshift surveys to investigate the relationship between the 81 low-redshift Lyα absorbers in our HST/GHRS survey and galaxies, superclusters, and voids. In a subsample of 46 Lyα absorbers located in regions where the February 8, 2000 CfA catalog is complete down to at least L ∗ galaxies, the nearest galaxy neighbors range from 100h −1 70 kpc to Mpc. Of these 46 absorbers, 8 are found in galaxy voids. After correcting for pathlength and sensitivity, we find that 22 ± 8 % of the Lyα absorbers lie in voids, which requires that at least some low-column density absorbers are not extended halos of individual bright galaxies. The number density of these clouds yields a baryon fraction of 4.5 ± 1.5 % in voids. The stronger Lyα absorbers (1013.2−15.4 cm−2) cluster with galaxies more weakly than galaxies cluster with each other, while the weaker absorbers (1012.4−13.2 cm−2) are more randomly distributed. The median distance from a