Evidence for Cluster Evolution from an Improved Measurement of the Velocity Dispersion and Morphological Fraction of Cluster 1324+3011 at Z = 0.76

We have carried out additional spectroscopic observations in the field of cluster Cl 1324+3011 at z = 0.76. Combined with the spectroscopy presented in Postman, Lubin & Oke (2001, AJ, 122, 1125), we now have spectroscopically confirmed 47 cluster members. With this significant number of redshifts, we measure accurately the cluster velocity dispersion to be 1016 −93 km s −1. The distribution of velocity offsets is consistent with a Gaussian, indicating no substantial velocity substructure. As previously noted for other optically-selected clusters at redshifts of z & 0.5, a comparison between the X-ray luminosity (Lx) and the velocity dispersion (σ) of Cl 1324+3011 implies that this cluster is underluminous in X-rays by a factor of ∼ 3−40 when compared to the Lx−σ relation for local and moderate-redshift clusters. We also examine the morphologies of those cluster members which have available high-angular-resolution imaging with the Hubble Space Telescope (HST). There are 22 spectroscopically-confirmed cluster members within the HST field-of-view. Twelve of these are visually classified as early-type (elliptical or S0) galaxies, implying an early-type fraction of 0.55 −0.14 in this cluster. Space Telescope Science Institute is operated by the Association of Universities for Research in Astronomy, Inc., under contract to the National Aeronautics and Space Administration.