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 Lyalpha absorbers in our Hubble Space Telescope GHRS survey and galaxies, superclusters, and voids. In a subsample of 46 Lyalpha absorbers located in regions where the 2000 February 8 CfA catalog is complete down to at least L* galaxies, the nearest galaxy neighbors range from 100 h(70)(-1) kpc to greater than 10 h(70)(-1). Mpc. Of these 46 absorbers, eight are found in galaxy voids. After correcting for path length and sensitivity, we find that 22%+/-8% of the Lyalpha 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 Lyalpha absorbers (10(13.2)-10(15.4) cm(-2)) cluster with galaxies more weakly than galaxies cluster with each other, while the weaker absorbers (10(12.4)-10(13.2) cm(-2)) are more randomly distributed. The median distance from a low-z Lyalpha absorber in our sample to its nearest galaxy neighbor (similar to500 h(70)(-1) kpc) is twice the median distance between bright galaxies in the same survey volume. This makes any purported "association" between these Lyalpha absorbers and individual galaxies problematic. The suggested correlation between Lyalpha absorber equivalent width (W) and nearest galaxy impact parameter does not extend to W less than or equal to 200 mAngstrom or to impact parameters greater than 200 h(70)(-1) kpc. Instead, we find statistical support for the contention that absorbers align with large-scale filaments of galaxies. The pair of sight lines, 3C 273 and Q1230+0115, separated by 0degrees.9 on the sky, provides an example of eight absorbers and seven galaxies aligned along a possible filamentary structure at least 20 h(70)(-1) Mpc long. While some strong (W > 400 mAngstrom) Lyalpha absorbers may be gas in the extended gaseous halos of individual galaxies, much of the local Lyalpha "forest" appears to be associated with the large-scale structures of galaxies and some with voids.