ar X iv : a st ro - p h / 97 09 13 6 v 1 1 5 Se p 19 97 Ly α absorbers arising in galaxy clusters

We present here new GHRS observations of Lyα 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 Lyα absorption features, with a mean redshift of < z >= 0.2648 ± 0.0002 and a velocity dispersion of 163 ± 57 km s −1. 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 −1. The spectrum of QSO 0850+4400, of poorer quality, reveals two Lyα absorption features at z = 0.0909500 ± 0.0000070 (which is just resolved) and z = 0.0948215 ± 0.0000090, separated by ∼ 1060 km s −1. 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 −1. The results of this work establish that Lyα 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 Lyα absorption lines being associated with individual galaxies also in groups and clusters. Moreover, the data shows clearly that line clustering takes place in the Lyα forest. Details about the GHRS spectra of 1545+2101 and 0850+4400 (see Fig. 1) will be presented elsewhere [3]. A standard Voigt profile fitting [1] to individual lines (or groups of them) shows the following features in each field: (a) 1545+2101: two lines are found at z = 0.2504707 ± 0.0000030 and z = 0.2522505 ± 0.0000016, with a velocity separation of ∼ 427 km s −1. Also present is a group of eight lines, whose redshift centroid is < z >= 0.2648 ± 0.0002 with a velocity dispersion of 163 ± 57 km s −1. (b) 0850+4400: two lines are detected at z = 0.0909500±0.0000070 and z = 0.0948215±0.0000090, with a velocity separation of 1062 ± 3 km s −1. A number of galaxies in these two fields have been observed spectroscopi-cally [2] to determine their possible connection to the absorption features detected on the QSOs spectra: (a) …