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Multiple Merging Events in the Double Cluster A3128/A3125
Multifiber spectroscopy has been obtained for 335 galaxies in the fieldof the double cluster A3128/A3125, using the 2dF multifiber positioneron the Anglo-Australian Telescope. When combined with previouslypublished results, a total of 532 objects in the double cluster now haveknown redshifts. We have also obtained a 20 ks Chandra ACIS-I image ofthe central 16'×16' of A3128 and radioimaging of the cluster with the Molonglo Observatory Synthesis Telescopeand the Australia Telescope Compact Array. The spatial/kinematicdistribution of redshifts in the field of A3128/A3125, when combinedwith the Chandra ACIS-I image of A3128, reveals a variety ofsubstructures present in the galaxy distribution and in the hotintracluster medium (ICM). The most striking large-scale feature in thegalaxy distribution is a relatively underpopulated redshift zone ~4000km s-1 on either side of the mean cluster velocity at ~17,500km s-1. We attribute this depletion zone to the effect of theextensive Horologium-Reticulum (H-R) supercluster, within whichA3128/A3125 is embedded. In addition to this large-scale feature,numerous smaller groups of galaxies can be identified, particularlywithin the underpopulated region within +/-4000 km s-1 of themean cluster redshift. Because of the large gravitational influence ofthe H-R supercluster, these groups arrive at A3128 with a high infallvelocity, well in excess of the local sound speed. Two of these groupsappear as elongated filaments in position-velocity diagrams, indicatingthat they are tidally distended groups that have been disrupted after aclose passage through A3128. In fact, A3125 itself appears to be in sucha postpassage condition. We have identified a primarynortheast-southwest merger axis connecting A3128 with A3125, along whichthe filaments are also oriented. In addition, the Chandra image revealsthat the X-ray emission is split into two components, each with verysmall core radii, that are separated by ~1 Mpc along thenortheast-southwest axis. We have combined the redshift, X-ray, andradio data to propose that the complex X-ray morphology revealed in theChandra image is likely the result of a hypersonic infall of arelatively small group into A3128. The group produces a major disruptionin the ICM as a result of its high infall velocity.
| A catalog of morphological types in 55 rich clusters of galaxies
Data are presented from a study of 55 rich clusters of galaxies. Thedata include positions, morphological types, estimated total magnitudes,bulge sizes, and ellipticities for about 6000 galaxies, as determinedfrom high scale photographic plates. Data reduction procedures aredescribed, and a brief analysis of cluster richness, which indicatesthat Abell richness classes are only rough indicators of total clustermembership, is included.
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