A SPATIAL, KINEMATICAL, AND DYNAMIC ANALYSIS OF ABELL-400

A detailed spatial, kinematical, and dynamical analysis is presented for the cluster A400, based on a nearly complete redshift survey of bright galaxies within 1 Mpc of the cluster center (88 of 92 galaxies in the field studied by Dressler). The structure of this cluster is shown to be rather complex. We identify a dispersed component with a high fraction of spiral galaxies at a velocity of 8200 km s-1, and a background group with a mean velocity of 13,400 km s-1. Although we consider a model in which A400 is comprised of a single cluster at a central location in velocity space on the order of 7000 km s-1, we propose that the main body of A400 is composed of at least two individual subclusters. If subclustering is ignored, the derived dispersion of the 88 galaxies with measured velocities within 4000 km s-1 of the bright dumbbell galaxy near the cluster center is 702 km s-1. When kinematic information is used to split A400 into likely subclusters, the velocity dispersions of the individual units which make up this cluster are on the order of 200-300 km s-1. If A400 is considered a single entity (including the dispersed high-velocity spiral-rich component), the inferred blue mass-to-light ratio is 1210 M./L.. If the cluster at 7000 km s-1 is taken to be a single entity, the inferred mass-to-light ratio is 615 M./L., which is still higher than is found for most rich clusters, If the primary sublusters which make up this low-velocity structure are considered separately, their derived mass-to-light ratios drop to roughly 300 M./L., in line with estimates for other groups and clusters. Application of a simple two-body dynamical model indicates that the two primary subclusters in A400 are bound to one another, and are presently infalling with relative velocity difference on the order of either 700 km s-1 or 2000 km s-1 depending on the projection angle of the system with respect to the plane of the sky. We argue that A400 is an example of a presently occurring merger, and that the individual components of the dumbbell galaxy were once individual D galaxies within the premerger subclusters. A400 harbors the unusual radio source 3C 75, which is associated with the bright central dumbbell galaxy. Each component of the dumbbell galaxy exhibits a twin radio jet; these jets bend in a common direction on the sky. The complex structure of 3C 75 might well be explained by our subcluster collision model. The Einstein X-ray image of A400 exhibits an apparent asymmetry in the central regions, with a major axis approximately aligned along a direction parallel to the initial bend of the wide-angle-tail radio sources. The inferred collisional velocity of the subclusters is large enough to cause the bending in the radio jets due to the motion of the D galaxies through the intracluster gas trapped in the potential wells of each subcluster.