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 toinvestigate the relationship between the 81 low-redshift Lya absorbers in ourHST/GHRS survey and galaxies, superclusters, and voids. In a subsample of 46Lya absorbers located in regions where the February 8, 2000 CfA catalog iscomplete down to at least L* galaxies, the nearest galaxy neighbors range from100kpc to >10 Mpc. Of these 46 absorbers, 8 are found in galaxy voids. Aftercorrecting for pathlength and sensitivity, we find that 22+-8% of the Lyaabsorbers lie in voids, which requires that at least some low-column densityabsorbers are not extended halos of individual bright galaxies. The numberdensity of these clouds yields a baryon fraction of 4.5+-1.5% in voids. The stronger Lya absorbers (10^{13.2-15.4} cm^-2) cluster with galaxies moreweakly than galaxies cluster with each other, while the weaker absorbers(10^{12.4-13.2} cm^-2) are more randomly distributed. The median distance froma low-z Lya absorber in our sample to its nearest galaxy neighbor (~500 kpc) istwice the median distance between bright galaxies in the same survey volume.This makes any purposed "association" between these Lya absorbers andindividual galaxies problematic. The suggested correlation between Lya absorberequivalent width (W) and nearest-galaxy impact parameter does not extend toW<200mA, or to impact parameters >200kpc. Instead, we find statistical supportfor the contention that absorbers align with large-scale filaments of galaxies.While some strong (W>400mA) Lya absorbers may be gas in the extended gaseoushalos of individual galaxies, much of the local Lya "forest" appears to beassociated with the large-scale structures of galaxies and some with voids.Comment: 56 pages, 13 .eps figures, 5 tables, accepted to Ap