The Gaseous Extent of Galaxies and the Origin of LY alpha Systems. II. Identification of a Group or Cluster of LY alpha Absorbing Galaxies at Z approximately equal to 0.26

Abstract

We report the identification of a group or cluster of galaxies at redshift z ~ 0.26 toward the quasi-stellar object (QSO) 1545+2101 that produces a complex of corresponding Ly alpha absorption systems. This observation demonstrates that at least some groups or clusters of galaxies produce Ly alpha absorption systems and that at least some Ly alpha absorption systems arise in groups or clusters of galaxies. Furthermore, this observation suggests that previous measurements may have significantly underestimated the degree of clustering of Ly alpha absorption systems on velocity scales of several hundred kilometers per second.