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

Abstract

In this paper, we use large-angle, nearby galaxy redshift surveys to investigate the relationship between the 81 low-redshift Lya absorbers in our HST/GHRS survey and galaxies, superclusters, and voids. In a subsample of 46 Lya 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 100kpc to >10 Mpc. Of these 46 absorbers, 8 are found in galaxy voids. After correcting for pathlength and sensitivity, we find that 22+-8% of the Lya 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 Lya absorbers (10^{13.2-15.4} cm^-2) cluster with galaxies more weakly than galaxies cluster with each other, while the weaker absorbers (10^{12.4-13.2} cm^-2) are more randomly distributed. The median distance from a low-z Lya absorber in our sample to its nearest galaxy neighbor (~500 kpc) is twice the median distance between bright galaxies in the same survey volume. This makes any purposed &#34;association&#34; between these Lya absorbers and individual galaxies problematic. The suggested correlation between Lya absorber equivalent width (W) and nearest-galaxy impact parameter does not extend to W<200mA, or to impact parameters >200kpc. Instead, we find statistical support for the contention that absorbers align with large-scale filaments of galaxies. While some strong (W>400mA) Lya absorbers may be gas in the extended gaseous halos of individual galaxies, much of the local Lya &#34;forest&#34; appears to be associated with the large-scale structures of galaxies and some with voids.