The gaseous extent of galaxies and the origin of Lyα absorption systems. II. Identification of a group or cluster of Lyα absorbing galaxies at z ≈ 0.26

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 Lyα absorption systems. This observation demonstrates that at least some groups or clusters of galaxies produce Lyα absorption systems and that at least some Lyα 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 Lyα absorption systems on velocity scales of several hundred kilometers per second.