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 recently presented by Postman, Lubin, & Oke, 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)(+126) km s(-1). The distribution of velocity of sets is consistent with a Gaussian, indicating no substantial velocity substructure. As previously noted for other optically selected clusters at redshifts of zgreater than or similar to0.5, a comparison between the X-ray luminosity (L-X) and the velocity dispersion (sigma) of Cl 1324+3011 implies that this cluster is underluminous in X-rays by a factor of similar to3-40 when compared with the L-X-sigma relation for local and moderate-redshift clusters. We also examine the morphologies of those cluster members that 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)(+0.17) in this cluster. This fraction is a factor of similar to1.5 lower than that observed in nearby rich clusters. Confirming previous cluster studies, the results for cluster Cl 1324+3011, combined with morphological studies of other massive clusters at redshifts of 0less than or equal tozless than or similar to1, suggest that the galaxy population in massive clusters is strongly evolving with redshift. This evolution implies that early-type galaxies are forming out of the excess of late-type ( spiral, irregular, and peculiar) galaxies over the similar to7 Gyr timescale.