THE FORMATION OF COMPACT-GROUPS OF GALAXIES .1. OPTICAL-PROPERTIES

The small crossing time of compact groups of galaxies (t(cr) H-0 less than or similar to 0.02) makes it hard to understand why they are obse rvable 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 appr oximately 1 Gyr. On this time scale, members may merge and/or other ga laxies 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)] comp act groups (HCG's): (1) CG's have a frequency distribution of members similar to that of HCG's; (2) CG's are almost-equal-to 10 times as den se as loose groups; (3) CG's have dynamical properties remarkably simi lar 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 b etween CG's and the surrounding rich loose group. Our model predicts t he frequency of occurrence of CG's. A preliminary analysis of 18 rich loose groups is consistent with the model prediction. We suggest furth er observational tests of the model.