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 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 approximately 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 almost-equal-to 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.