The Local Ly-alpha forest: Association of absorbers with galaxies, voids, and superclusters

Abstract

We describe recent discoveries of low column density [N(HI) < 10^14.5 cm^-2] H I Ly-alpha absorbers made with the Hubble Space Telescope (HST), which have allowed us a first look at gas in local intergalactic space between us and the &#34;Great Wall&#34;. Despite the mere 2.4m aperture of HST, these new observations allow us to detect absorbers with column densities, N(HI) \approx 10^12.5 cm^-2, as low as those found using Keck/HIRES at high-z. Owing to the proximity of these absorbers to the Earth, the 197 absorbers in our combined GHRS + STIS sample (Penton, Stocke, & Shull 2001) provide our best view of the relationship between these absorbers and galaxies, voids, and supercluster filaments. Unlike previous results based on galaxy surveys near higher-N(HI) absorbers, we find no evidence that these lower-N(HI) absorbers are extended galaxy halos. Rather, the majority (78%) are associated with large-scale filamentary structures of galaxies, while 22% are found in galaxy &#34;voids&#34;. Since these Ly-alpha absorbers are currently the only baryons detected in the voids, we use their properties to estimate that the voids contain 4.5 (+/-1.5)% of the Universal baryon density.