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Formalism and quality of a proper motion link with extragalactic objects for astrometric satellite missions
The accuracy of the link of the proper motion system of astrometricsatellite missions like AMEX and GAIA is discussed. Monte-Carlo methodswere used to simulate catalogues of positions and proper motions ofquasars and galaxies to test the link. The main conclusion is, thatfuture satellite missions like GAIA may be ``self-calibrated'' by theirmeasurements of QSOs, while additional measurements from radio stars orHST-data are needed to calibrate the less deep reaching astrometricsatellite missions of AMEX type.
| 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.
| The Horologium-Reticulum supercluster of galaxies
The existence of a supercluster at approximately 18,000 km/s in theHorologium-Reticulum region is demonstrated. The principal constituentsof the H-R supercluster are two rich clusters, Cl 0342-538 and 0329-527,one of which has an adjacent loose enhancement of galaxies, Cl 0326-537.A simple model indicates that the two main clusters of the H-Rsupercluster are at present approaching each other and will collidewithin 0.5 of a Hubble time. It is noted that a bridge (or filament) ofgalaxies connects Cl 0342-538 to the loose enhancements at Cl 0326-537.It is found that the position angles of the five bridge galaxies aresignificantly aligned along the direction of the bridge. A number offoreground groups are detected. One such system at 13,700 km/s coversmost of the survey region, exhibits a velocity dispersion of only 210km/s, and is similar to the G47 system observed in front of the Herculessupercluster. Several regions completely devoid of galaxies areidentified.
| 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.
| New nebulae and nebulous stars.
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