Annu. Rev. Astron. Astrophys. 1979. 17: 135-87 Copyright © 1979 by Annual Reviews. All rights reserved

6.3 Compact Groups

Because of possible interpenetrations of nonluminous massive envelopes in compact groups of galaxies, these groups provide a severe challenge to any theory of gravitational encounters between galaxies. Rose (1979) and Rose & Graham (1979) have recently reviewed the various hypotheses for the origin of such groups. A possible problem with these groups is their short lifetimes; if the member galaxies really do have massive envelopes, the galaxies should coalesce rapidly due to dynamical friction. Alternatively, perhaps these compact groups survive just because their galaxies have abnormally small envelopes.

Reviews of compact groups have been given by Karachentsev (1966) and by Burbidge & Sargent (1971); Rose (1977) has conducted a new survey for such groups. The problem of discrepant redshifts has long plagued the dynamical study of compact groups. Rose (1977) suggests that they can be explained simply on the basis of chance projections, but Nottale & Moles (1978) feel that discrepant cases are too frequent to be explained in this way.

For many of the groups, however, the velocity dispersion appears normal, and the virial theorem should apply. Regardless of the origin of these compact configurations, one would predict small mass-to-light ratios for those groups because the spatial separations between the member galaxies are too small to sample the gravitational potential of extended, nonluminous envelopes. This prediction appears to be consistent with the available information. The median M / L_B for the compact groups listed by Burbidge & Sargent is 30 (an exact value cannot be specified because the magnitude system was not fully described), while Rose & Graham find values of only 2.3 and 12.2 for two southern compact groups.