HST /COS Lyman-alpha Absorbers in Cosmic Voids

Abstract: We investigate the spatial distribution of Lyman-$\alpha$ (Ly $\alpha$) absorbers within cosmic voids. We create a catalogue of cosmic voids in Sloan Digital Sky Survey Data Release 7 (SDSS DR7) with the VoidFinder algorithm of the Void Analysis Software Toolkit (VAST). Using the largest catalogue of low-redshift (z $\leq$ 0.75) IGM absorbers to date, we identify 392 Ly $\alpha$ absorbers inside voids. The fraction of Ly $\alpha$ absorbers inside voids (65 per cent) is comparable to the volume filling fraction of voids (68 per cent), and significantly greater than the fraction of galaxies inside voids (21 per cent). Inside voids, the spatial distribution of Ly $\alpha$ absorbers differs markedly from that of galaxies. Galaxy density rises sharply near void edges, while Ly $\alpha$ absorber density is relatively uniform. The radial distribution of Ly $\alpha$ absorbers inside voids differs marginally from a random distribution. We find that lower column density Ly $\alpha$ absorbers are more centrally concentrated inside voids than higher column density Ly $\alpha$ absorbers. These results suggest the presence of two populations of Ly $\alpha$ absorbers: low column density systems that are nearly uniformly distributed in the interiors of voids and systems associated with galaxies at the edges of voids.