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|The ESO nearby Abell cluster survey. VIII. Morphological and spectral classification of galaxies|
We determine the morphological types of 2295 galaxies from the ESONearby Abell Cluster Survey (ENACS) from CCD images obtained with theDutch telescope on La Silla. A comparison with morphological types fromthe literature for 450 of our galaxies shows that the reliability of ourclassification is quite comparable to that of other classifiers. Werecalibrate the ENACS spectral classification with the new morphologicaltypes, and find that early- and late-type galaxies can be distinguishedfrom their spectra with 83% reliability. Ellipticals and S0 galaxies canhardly be distinguished on the basis of their spectra, but late spiralscan be classified from the spectrum alone with more than 70%reliability. We derive pseudo-colors and linestrengths from the ENACSspectra for the galaxies of different morphological types. We considerthe bright (MR ≤ -20) and faint (MR > -20)subsets of the galaxies without emission lines (non-ELG) separately. Wefind a strong and significant correlation between the average color andthe average strength of the metal absorption lines. The averagemetallicity decreases and the average color gets bluer towards laterHubble type. Also, the faint galaxies in each morphological class arebluer and less metal-rich than their brighter counterparts, whichextends the well-established color-magnitude relation of early-typegalaxies to (late) spirals. In view of these very strong global trends,the colors and metallicities of faint S0 galaxies and bright earlyspirals are remarkably similar. The bright early spirals may, onaverage, have somewhat stronger Hδ absorption than the othergalaxies, which could be due to recent starformation. The galaxies withemission lines (ELG) have a bluer spectral continuum than the non-ELG,and the amount of blueing hardly depends on morphological type. Thefraction of ELG depends strongly on morphological type (varying from4±1% for ellipticals to 59±4% for late spirals), but foreach of the morphological types it varies very little with projecteddistance from the cluster center.
|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 ESO Nearby Abell Cluster Survey. V. The catalogue: Contents and instructions for use|
We present the catalogue resulting from the ESO Nearby Abell ClusterSurvey (the ENACS), which contains redshifts and magnitudes for 5634galaxies in the directions of 107 rich, nearby southern Abell clustercandidates. We describe the contents of the catalogue and discuss theresults of a comparison between the ENACS catalogue and the COSMOSGalaxy Catalogue. When cross-correlating the two catalogues we findthat, at least in the areas of the ENACS clusters, the completeness ofthe COSMOS catalogue is somewhat lower than was estimated previously forthe carefully analyzed and well-calibrated part of the COSMOS catalogueknown as the Edinburgh-Durham Southern Galaxy Survey (EDSGC). The galaxypositions in the COSMOS and ENACS catalogues are found to be on the samesystem to within about one arcsecond. For the clusters for which thephotometry in the ENACS and COSMOS catalogues is based on the samesurvey plates, the two magnitude scales agree very well. We confirm thatthe photometric calibration in the EDSGC subset of the COSMOS catalogueis of higher quality than in the EDSGC complement. The ENACS galaxysamples are unbiased subsets of the COSMOS catalogue as far as theprojected galaxy distribution is concerned, except in only a few cases.We summarize how the ENACS galaxy samples are subsets of the COSMOScatalogues in the ENACS apertures, with respect to magnitude. For theENACS catalogue as a whole, we describe the apparent incompleteness atfaint magnitudes and towards higher redshifts. Finally, we provide somedetailed information about the ENACS catalogue that is essential for itsproper statistical use and we summarize some facts that must beremembered when selecting subsets of galaxies from it. Based onobservations collected at the European Southern Observatory (La Silla,Chile).
|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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