A dynamical study of optically selected distant clusters

We present a programme of spectroscopic observations of galaxies in a sampleof optically-selected clusters taken from the catalogue of Couch et al (1991).Previous ROSAT observations of these clusters have shown them to have lowerX-ray luminosities, given their optical richness, than might be expected on thebasis of local samples. In the present paper we extend this work by determiningvelocity dispersions of a subsample of the clusters. We confirm the dynamicalreality of all but one of the original sample, and find velocity dispersionscomparable with present-day clusters of equivalent comoving space density.Thus, in the context of the $L_X-\sigma$ 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 thegravitational potential. If they are, the X-ray luminosities measured in Boweret al., 1994 (Paper I), would then imply an implausibly low efficiency of X-raygeneration. Alternatively, the discrepancy could be explained if the clusterswere systems of lower virial temperature, in which the apparent velocitydispersion is inflated by an infalling, unrelaxed halo. This might resulteither from an increase with redshift in the infall rate for clusters, or fromthe preferential selection of clusters embedded in filaments oriented along theline of sight. Since clusters with similar properties can be found in localoptically selected catalogues, we suggest that the latter explanation is morelikely.