THE GASEOUS EXTENT OF GALAXIES AND THE ORIGIN OF LY-ALPHA ABSORPTION SYSTEMS .2. IDENTIFICATION OF A GROUP OR CLUSTER OF LY-ALPHA ABSORBINGGALAXIES AT Z-APPROXIMATE-TO-0.26

We report the identification of a group or cluster of galaxies at reds hift z approximate to 0.26 toward the quasi-stellar object (QSO) 15452101 that produces a complex of corresponding Ly alpha absorption syst ems. This observation demonstrates that at least some groups or cluste rs of galaxies produce Ly alpha absorption systems and that at least s ome Ly alpha absorption systems arise in groups or clusters of galaxie s. Furthermore, this observation suggests that previous measurements m ay have significantly underestimated the degree of clustering of Ly al pha absorption systems on velocity scales of several hundred kilometer s per second.