Structure and Dynamics of the Coma Cluster

We examine the structure and dynamics of the galaxies in the Coma cluster using a catalog of 552 redshifts including 243 new measurements obtained with the Hydra multifiber spectrograph at KPNO The velocity distribution for the 465 cluster members is clearly non-Gaussian. A new test for localized variations in the velocity distribution shows highly significant structure associated with the group of galaxies around NGC 4839, 40' SW of the cluster core. We apply a mixture-modeling algorithm to the galaxy sample and obtain a robust partition into two subclusters: the main Coma cluster centered on NGC 4874, with barcz = 6853 km s-1 and σcz = 1082 km s-1, and a group centered on NGC 4839, with barcz = 7339 km s-1 and σcz = 329 km s-1. We use this partition to examine the system's dynamics. We find that the velocity dispersion of the late-type galaxies in the main cluster is very close to 2½ times that of the early-type galaxies, suggesting that the late types are freely falling into a largely virialized cluster core dominated by early types. We obtain a virial mass for the main cluster of 0.9 × 1015 h-1 Msun, in close agreement with the estimates derived from recent X-ray data. The mass of the NGC 4839 group is found to be 0.6 × 1014 h-1 Msun, or about 5%-10% the mass of the main cluster, in accord with their relative richnesses and X-ray luminosities. Assuming the main cluster and the NGC 4839 group follow a linear two-body orbit, the favored solution has the two clusters lying at 74° to the line of sight at a true separation of 0.8 h-1 Mpc and moving together at 1700 km s-1. Closer examination of the cluster core reveals an ongoing merger between two subclusters centered in projection on the dominant galaxies NGC 4874 and NGC 4889 but offset in velocity by 300 and 1100 km s-1, respectively. Combining these results with X-ray and radio observations, and an interpretation of the presence or lack of an extended halo around the dominant galaxies NGC 4874, NGC 4889, and NGC 4839, we develop a merger history for the Coma cluster. We suggest that the cD NGC 4839 is at the center of the group in which it formed, and that this group is just beginning to penetrate the Coma cluster, into which it is falling from the direction of Abell 1367 along the Great Wall. We argue that the radio halo of Coma and the disturbed X-ray emission in its core are the result of the ongoing merger between the main cluster and a group dominated by NGC 4889. This group has now been partially disrupted, ejecting NGC 4889. The position and apparent halo of NGC 4874 indicate that it was the original dominant galaxy of the main Coma cluster, though it may now have been dislodged from the bottom of the potential well.