The kinematics of Abell clusters

We have assembled a sample of 69 nearby Abell clusters (0.01 ≤ z ≤ 0.15) which have 10 or more measured redshifts within 1.5h^-1 Mpc of the cluster center. The data include 359 new galaxy redshifts. For each system, we display the velocity histogram and galaxy positions used to determine cluster membership. We derive velocity dispersions for 65 of the clusters. The median velocity dispersion for the sample is 744 km s^-1. From the distribution of cluster velocity dispersions, we obtain the following results. (1) The shape of the distribution for the subset of 49 richness class R ≥ 1 clusters does not match the predictions of cold dark matter (CDM) models with biased galaxy formation for any biasing parameter b. The only CDM models consistent with the median (805 km s^-1) and maximum (1330 km s^-1) dispersion we observe are those with b ∼ 1.6-2.0 and in which clusters are identified in three dimensions. (2) The dispersions of clusters in our sample are generally smaller than those of more distant Butcher-Oemler-type systems (0.31 ≤ z ≤ 0.55). Although cluster evolution might account for this effect, richer systems and/or contaminated fields may be preferentially selected at large z. (3) The median dispersion of the subset of 25 clusters which contain a "cD" galaxy (773 km s^-1) is similar to that of the non-cD clusters (661 km s^-1). (4) A substantial fraction of the cD galaxies (eight of 19) have velocities significantly different from the mean of their parent clusters. The last two results suggest that the formation of a cD galaxy is dominated by its local environment. In principle, information about the dynamical states of clusters could be derived from the shapes of individual velocity histograms. Monte Carlo simulations show, however, that the velocity distributions of clusters in our data set are insufficiently sampled to allow extraction of moments higher than the dispersion.