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For refcode 1989A&A...225..291W:
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Copyright by European Southern Observatory (ESO). Reproduced by permission
1989A&A...225..291W The frequency of chance alignments of galaxies in loose groups D.G. Walke and G.A. Mamon Department of Physics, New York University, 4 Washington Place, New York, NY 10003, USA Raritan Valley Community College, P.O. Box 3300, Somerville, NJ 08876, USA DAEC, Observatoire de Meudon, F-92195 Meudon Cedex, France UFR de Physique, Universite de Pairs VII, Place Jussieu, F-75005 Paris, France Received November 8, 1988, accepted April 17, 1989 Summary. We derive semi-analytical expressions for the frequency of chance occurrence of isolated subgroups (using Hickson's 1982 definition) within groups of galaxies. Our analysis is checked with Monte-Carlo simulations that are 1000 times costlier. We generalize our model to investigate the issues of contamination by interlopers and compactness of the subgroups. Using the NBG galaxy and grouping catalog as a reference, we show that contamination has a small effect on the frequency of isolated subgroups, but explains the relatively large fraction of discordant redshifts in Hickson's compact groups. We confirm the high frequencies of chance alignments found in earlier dynamical simulations of groups. We also find that the typical NBG group will contribute enough to produce, by chance alignments of galaxies, compact groups strictly meeting Hickson's selection criteria. However, since Hickson's sample is incomplete at its compactness limit, the typical NBG group will contribute negligibly to producing compact groups as compact as Hickson's by chance alignments. Nevertheless, we show that a few loose groups, as well as small clusters and the Virgo cluster contribute disproportionately more than the typical NBG group. Furthermore, we show that binaries are important in NBG groups. We argue that the combination of these two effects account for roughly half of Hickson's compact groups as chance alignments of galaxies within intermediate density groups and small clusters. Key words: analytical methods - clusters of galaxies
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