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For refcode 1988ApJ...331L..69H:
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1988ApJ...331L..69H THE NATURE OF COMPACT GROUPS OF GALAXIES PAUL HICKSON Department of Geophysics and Astronomy, University of British Columbia AND HERBERT J. ROOD P.O. Box 1330, Princeton, NJ 08542 Received 1988 April 19; accepted 1988 May 27 ABSTRACT We calculate, by Monte Carlo numerical simulation, the probability for the chance occurrence of four galaxies projected on the sky satisfying the Hickson isolation criterion within a loose group of eight members. For the models which match most closely the size and galaxy multiplicity function of observed groups, this chance occurrence is found to be smaller by a factor of ~100 than the value obtained previously by Mamom from numerical simulations of dynamical models of groups. This and other direct independent observational results from the literature constitute strong evidence that nearly all of the Hickson compact groups are real physical systems. We conclude that the tendency for the spiral fraction of a compact group to be larger than the value inferred from the galaxy morphology-group density relation of rich clusters and loose groups is a real physical effect indicating that galaxy morphology depends strongly on a second parameter, which we suggest is the velocity dispersion of a system. We demonstrate that spiral fraction decreases monotonically with increasing velocity dispersion along a sequence containing loose groups, compact groups, and rich clusters. Subject heading: galaxies: clustering
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