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A 2572 and HCG 94 - galaxy clusters but not as we know them: an X-ray case study of optical misclassifications
We present the results of a spectral-imaging analysis of X-ray dataobtained with the Position Sensitive Proportional Counter aboard theROSAT Observatory in a 32-ks pointed observation of Hickson's CompactGroup (HCG) £94. Besides HCG 94, A2572, a richness class 0Abell cluster, is also contained in the central region of the field ofview. Both systems are at a redshift of z~0.04 and are falling towardeach other at a velocity of about 1000 km s^-1. Their three-dimensionalspatial separation is probably of the order of an Abell radius; however,as yet, no clear signs of dynamical interaction are discernible in theX-ray data. We find HCG 94's gas temperature and unabsorbed X-rayluminosity to be far too high for a galaxy group, thereby confirming theclaim of Ebeling, Voges & Bohringer that HCG 94 should be classifiedas a galaxy cluster. The opposite is true for the Abell cluster A2572,the optical richness of which has been overestimated due to theinclusion of HCG 94. In the X-ray, A2572 appears at first sight like atypical binary cluster with two equally massive and X-ray-brightsubclusters in the process of merging. However, the available X-ray,optical, and radio data strongly suggest that A2572 proper is, in fact,merely a loose group of galaxies, while the second component is a muchricher and more distant cluster seen in superposition. A deprojectionanalysis shows HCG 94 to host a moderate cooling flow; this picture issupported by a radial increase in the column density of absorbingmaterial and a decrease in the gas temperature toward the clustercentre. HCG 94's total gravitating mass is much higher than what couldbe anticipated from its appearance in the optical. Our findings henceunderline the need for X-ray-selected cluster samples. For all threeclusters studied in this paper we find the baryon fraction to rise withradius and reach values of 15 to 30 per cent at the outer edge of ourstudy regions. If any of these values is to be taken as representativeof the overall baryon fraction of the Universe, then this resultrequires the latter to be open with Omega_0<0.35 if a conflict withthe baryon density derived from nucleosynthesis calculations is to beavoided.
| Accurate Optical Positions of Arakelian Galaxies
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1981AJ.....86..820K&db_key=AST
| Galaxies of high surface brightness
Two lists are presented which contain 621 galaxies whose surfacebrightness, as derived from their apparent magnitudes, is at least 22.0magnitudes from an area of 1 sq arcsec. The lists were compiled in anattempt to verify observationally a possible correlation between surfacebrightness and nuclear activity. Four percent of all the galaxies in anarea of 4.5 sr at declinations higher than -3 deg and galactic latitudesgreater than 20 deg are listed, including 30 Markarian, 29 Zwicky, and 7blue Haro galaxies. A morphological study of 130 of the galaxiesindicates that about half are elliptical or lenticular, 50 are compactor peculiar, and that there is an excess of elliptical and lenticularobjects in comparison with a random sample. Notes on the morphologicaltypes and colors of the galaxies are provided along with identificationcharts.
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