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For refcode 1994AJ....107..868D:
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1994AJ....107..868D THE FORMATION OF COMPACT GROUPS OF GALAXIES. I. OPTICAL PROPERTIES ANTONALDO DIAFERIO Universita di Milano, Dipartimento di Fisica, Via Celoria 16,20133, Milano, Italy and Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, Massachusetts 02138 Electronic mail: diaferio@cfa.harvard.edu MARGARET J. GELLER Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, Massachusetts 02138 Electronic mail: mjg@cfa.harvard.edu MASSIMO RAMELLA Osservatorio Astronomico di Trieste, Via G. B. Tiepolo II, 34131 Trieste, Italy Electronic mail: ramella@cfa:harvard.edu Received 1993 September 13; revised 1993 November 17 ABSTRACT The small crossing time of compact groups of galaxies (t_cr_ H_0_ <~0.02) makes it hard to understand why they are observable at all. Our dissipationless N-body simulations show that within a single rich collapsing group compact groups of galaxies continually form. The mean lifetime of a particular compact configuration is ~1 Gyr. On this time scale, members may merge and/or other galaxies in the loose group may join the compact configuration. In other words, compact configurations are continually replaced by new systems. The frequency of this process explains the observability of compact groups. Our model produces compact configurations (CG's) with optical properties remarkably similar to Hickson's [ApJ, 255,382(1982)] compact groups (HCG's): (1) CG's have a frequency distribution of members similar to that of HCG's; (2) CG's are 10 times as dense as loose groups; (3) CG's have dynamical properties remarkably similar to those of HCG's; (4) most of the galaxy members of CG's are not merger remnants. The crucial aspect of the model is the relationship between CG's and the surrounding rich loose group. Our model predicts the frequency of occurrence of CG's. A preliminary analysis of 18 rich loose groups is consistent with the model prediction. We suggest further observational tests of the model.
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