Annu. Rev. Astron. Astrophys. 1982. 20: 547-85 Copyright © 1982 by Annual Reviews. All rights reserved

4.6 Groups of Galaxies

Bright, centrally located galaxies have been found in 23 poor clusters (Morgan et al. 1975, Albert et al. 1977). Massive galaxies can form in these environments through galaxy mergers, since timescales for two-body relaxation and dynamical friction are an order of magnitude shorter in small groups (and subclusters) than in rich, evolved clusters (see Section 1.2). Bahcall (1980) found that the Morgan groups exhibit a smooth continuation in cluster parameters (central density, richness, and spiral fraction) from rich clusters. Thuan & Romanishin (1981) showed that the cores of central, bright galaxies in both groups and clusters are similar to normal ellipticals, except that they are larger and have lower surface brightness. However, using surface brightness profiles, they demonstrated that the brightest galaxies in groups differ significantly from the cD galaxies in rich clusters in that those in groups are not observed to have extended envelopes.

X-ray sources have been identified with groups containing central bright galaxies (Schwartz et al. 1980, Kriss et al. 1980, Jones et al. 1979). The X-ray luminosities of these groups range from 5 x 10⁴¹ to 10⁴⁴ erg s^-1. For 7 groups, Cioffi et al. (private communication) have measured X-ray core radii in the range 130-250 kpc. X-ray temperatures for AWM7 (Mushotzky & Smith 1980), MKW3S, AWM4, and MKW4 (Cioffi et al.) lie between 1 and 3 keV. The spectrum of AWM7 also contains a high-temperature component (Mushotzky & Smith 1980). For the observed central gas densities of ~ 3 x 10^-3 cm^-3 (Cioffi et al.) and velocity dispersions exceeding 150 km s^-1, ram-pressure stripping of spirals should be effective (Gunn & Gott 1972). [Stauffer & Spinrad (1980) obtained = 622 km s^-1 from 11 galaxies in AWM7 and = 397 km s^-1 from 5 galaxies in AWM5.] However, Bahcall (1980) found a high spiral fraction (50 ± 10%) for the Morgan groups. This suggests either that the formation of these groups is recent (2 x 10⁹ yr) and stripping is occurring but star formation has not yet ceased; or that the gas-replenishment rates are sufficiently high to inhibit ram-pressure stripping (Gisler 1979); or that the spiral galaxies reside preferentially in outer regions where sweeping is not as effective as in the core.