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Session 122 - Galaxies, Clusters of Galaxies & IGM.
Oral session, Saturday, January 10
Jefferson,
I explore a procedure for the detection and quantification of substructure in the projected positions of galaxies in clusters. The method is first tested by application to the 56 well-studied galaxy clusters that make up the morphological sample of Dressler (1980). This method is then applied to a much larger sample of 119 Abell clusters which includes all Abell clusters with distance class \leq4 and richness class >0 with \vert b \vert >30. Galaxy positions are obtained from digitized copies of the POSS I plates carried out by the Minnesota Automated Plate Scanner project. Adaptive-kernel contour maps are constructed for each sample cluster and possible secondary peaks identified within one Abell radius (1.5h^-1 Mpc with H_0=100h km s^-1 Mpc^-1) of the cluster center. Two tests for substructure, one parametric and one nonparametric, are then applied to the galaxy positions and the results are compared. After removal of background/foreground groups determined by applying a K-S test to the distribution of apparent galaxy magnitudes, 64% \pm 15% of the large cluster sample is found to contain significant substructure. Radial number-density profiles are calculated for each of the sample clusters using a maximum penalized likelihood estimator. An average core radius of 112 \pm 75h^-1 kpc is obtained. Inside of 1 Mpc, the space density is found to vary as \rho \propto r^-1.9 \pm 0.3. The large fraction of clusters with presently-detectable substructure, as well as the shallow space-density profiles, are used to argue that rich clusters of galaxies are still in the process of formation during the present epoch through accretion of smaller groups of galaxies and are not in general well described by equilibrium models. With the assumption that substructure in the projected galaxy distribution is erased on a time scale of 4 cluster crossing times, then Ømega \gtrsim 0.4. On the other hand, if Ømega_0 \approx 0.2, as currently predicted, then substructure in the projected galaxy positions is erased on average in 6 to 7 cluster crossing times.
