Lyman-alpha absorbers arising in galaxy clusters

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We present here new GHRS observations of Lyman-alpha absorption lines associated with groups or clusters toward the QSOs 1545+2101 and 0850+4400. In the first case we have identified at least eight distinct Lyman-alpha absorption features, with a mean redshift of <z> = 0.2648 ± 0.0002 and a velocity dispersion of 163 ± 57 km/s. We have also identified a group or cluster of galaxies in the vicinity of this QSO with a mean redshift of <z> = 0.2643 ± 0.0004 and velocity dispersion 223 ± 91 km/s. The spectrum of QSO 0850+4400, of poorer quality, reveals two Lyman-alpha absorption features at z=0.0909500 ± 0.0000070 (which is just resolved) and z=0.0948215 ± 0.0000090, separated by 1060 km/s. A group or cluster of galaxies is also present in the vicinity of the QSO line-of-sight with a mean redshift <z>=0.0901 ± 0.0007 and velocity dispersion of 530 ± 200 km/s. The results of this work establish that Lyman-alpha absorption can occur in denser than average galaxy environments, and that it arises in discrete objects spanning a velocity range similar to that of the cluster galaxies. Although a one-to-one relationship between absorbers and galaxies is difficult to establish in such a dense environment, the results obtained here are indeed consistent with the Lyman-alpha absorption lines being associated with individual galaxies also in groups and clusters. Moreover, the data shows clearly that line clustering takes place in the Lyman-alpha forest.