Multiple Merging Events in the Double Cluster A3128/A3125

Multifiber spectroscopy has been obtained for 335 galaxies in the field of the double cluster A3128/A3125, using the 2dF multifiber positioner on the Anglo-Australian Telescope. When combined with previously published results, a total of 532 objects in the double cluster now have known redshifts. We have also obtained a 20 ks Chandra ACIS-I image of the central 16′ × 16′ of A3128 and radio imaging of the cluster with the Molonglo Observatory Synthesis Telescope and the Australia Telescope Compact Array. The spatial/kinematic distribution of redshifts in the field of A3128/ A3125, when combined with the Chandra ACIS-I image of A3128, reveals a variety of substructures present in the galaxy distribution and in the hot intracluster medium (ICM). The most striking large-scale feature in the galaxy distribution is a relatively underpopulated redshift zone ∼4000 km s -1 on either side of the mean cluster velocity at ∼17,500 km s -1. We attribute this depletion zone to the effect of the extensive Horologium-Reticulum (H-R) supercluster, within which A3128/ A3125 is embedded. In addition to this large-scale feature, numerous smaller groups of galaxies can be identified, particularly within the underpopulated region within ±4000 km s -1 of the mean cluster redshift. Because of the large gravitational influence of the H-R supercluster, these groups arrive at A3128 with a high infall velocity, well in excess of the local sound speed. Two of these groups appear as elongated filaments in position-velocity diagrams, indicating that they are tidally distended groups that have been disrupted after a close passage through A3128. In fact, A3125 itself appears to be in such a postpassage condition. We have identified a primary northeast-southwest merger axis connecting A3128 with A3125, along which the filaments are also oriented. In addition, the Chandra image reveals that the X-ray emission is split into two components, each with very small core radii, that are separated by ∼1 Mpc along the northeast-southwest axis. We have combined the redshift, X-ray, and radio data to propose that the complex X-ray morphology revealed in the Chandra image is likely the result of a hypersonic infall of a relatively small group into A3128. The group produces a major disruption in the ICM as a result of its high infall velocity.