A DYNAMICAL STUDY OF OPTICALLY SELECTED DISTANT CLUSTERS

We present a programme of spectroscopic observations of galaxies in a sample of optically selected clusters taken from the catalogue of Couc h et al, Previous ROSAT observations these clusters have shown them to have lower X-ray luminosities, given their optical richness, than mig ht be expected on the basis of local samples. In the present paper we extend this work by determining velocity dispersions of a subsample of the clusters. We confirm the dynamical reality of all but one of the original sample, and find velocity dispersions comparable to those of present-day clusters of equivalent comoving space density. Thus, in th e context of the L-x-sigma relation for present-day clusters, there X- ray luminosity. A key question is whether the high velocity dispersion s are indicative of the gravitational potential. If they are, the X-ra y luminosities measured by Bower et al. would then imply implausibly l ow efficiency of X-ray generation, Alternatively, the discrepancy coul d explained if the clusters are systems of low virial temperature, in which the apparent velocity dispersion is inflated by an infalling, un relaxed halo. By co-adding our sample, we are able consider multi-comp onent fits to the velocity distribution and to demonstrate evidence fo r large infalling population. This might result either from an increas e with redshift in the infall rate for clusters, or from the preferent ial selection of clusters embedded in filaments oriented along the lin e of sight, Since clusters with similar properties can be found in loc al optically selected catalogues, we suggest that the latter explanati on is more Likely.