A dynamical study of optically selected distant clusters

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Abstract
We present a programme of spectroscopic observations of galaxies in a sample of optically selected clusters taken from the catalogue of Couch et al. Previous ROSAT observations of these clusters have shown them to have lower X-ray luminosities, given their optical richness, than might 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 the context of the Lx−σ relation for present-day clusters, there is evidence for a higher velocity dispersion at fixed X-ray luminosity. A key question is whether the high velocity dispersions are indicative of the gravitational potential. If they are, the X-ray luminosities measured by Bower et al. would then imply an implausibly low efficiency of X-ray generation. Alternatively, the discrepancy could be explained if the clusters are systems of low virial temperature, in which the apparent velocity dispersion is inflated by an infalling, unrelaxed halo. By co-adding our sample, we are able to consider multi-component fits to the velocity distribution and to demonstrate evidence for a large infalling population. This might result either from an increase with redshift in the infall rate for clusters, or from the preferential selection of clusters embedded in filaments oriented along the line of sight. Since clusters with similar properties can be found in local optically selected catalogues, we suggest that the latter explanation is more likely.
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