The Local Lyα Forest: Association of Absorbers with Galaxies, Voids and Superclusters

We describe recent discoveries of low column density (NHI ≤ 10 cm) H I Lyα 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 “Great Wall”. Despite the mere 2.4m aperture of HST, these new observations allow us to detect absorbers with column densities, NHI ≈ 10 12.5 cm, 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 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-NHI absorbers, we find no evidence that these lower-NHI absorbers are extended galaxy halos. Rather, the majority (78%) are associated with large-scale filamentary structures of galaxies, while 22% are found in galaxy “voids”. Since these Lyα 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.