Annu. Rev. Astron. Astrophys. 1997. 35: 357-88 Copyright © 1997 by Annual Reviews. All rights reserved

4. DYNAMICAL PROPERTIES

The first studies of individual compact groups (Burbidge & Burbidge 1959, 1961b, 1961c, Burbidge & Sargent 1971, Rose & Graham 1979, Kirshner & Malumuth 1980) indicated short dynamical times and mass-to-light ratios intermediate between those of galaxies and rich clusters. However, because of the small number of galaxies, estimates of the space velocities and physical separations of galaxies in individual groups are highly uncertain. Meaningful dynamical conclusions about systems containing only four or five galaxies requires statistical analysis of large homogeneous samples.

By 1992, velocities had been measured for almost all 462 galaxies in the HCG catalog (Hickson et al. 1992). The distribution of galaxy velocities, relative to the median of each group, is shown in Figure 2. It can be seen that the majority of velocities fall within a roughly Gaussian distribution of standard deviation ~ 250 km s^-1 (the sharp peak at zero velocity results from the use of the median). This characteristic velocity is quite similar to velocity dispersions found in loose groups, and much smaller than typical velocity dispersions in rich clusters. In addition to the Gaussian core, a flat component is seen in the velocity distribution of Figure 2. This is expected as some galaxies which are not physically related to the group will appear projected on the group by chance. This component contains about 25% of the total number of galaxies. Whether of not chance projection can account for such a large number of "discordant" galaxies is still a matter of some debate, and is discussed further in Section 6.1.

Figure 2. Velocity distribution of galaxies in compact groups. The figure shows the distribution of the difference between the observed galaxy radial velocity and the median velocity of galaxies in the group to which it belongs, for 410 galaxies in the HCG catalog. Most galaxies (77%) have velocity differences less than 500 km s-1 from the median.

For a system of characteristic linear size R and internal velocity V, a characteristic dynamical time is t_d = R/V. A characteristic mass density is = 1/G t_d², from which one can estimate the total mass within the region occupied by the galaxies. After removing galaxies whose velocities differ from the group median by more that 1000 km s^-1, Hickson et al. (1992) found that t_d ~ 0.02 H₀^-1 and obtained a mean mass-to-light ratio of 50h (solar units) for the HCGs. Similar values were found for several Shakhbazian groups (Tikhonov 1986, Amirkhanian & Egikian 1987, Amirkhanian 1989, Lynds et al. 1990, Amirkhanian et al. 1991). Since the mass-to-light ratios of individual HCG galaxies is on the order of 7h (Rubin et al. 1991), the galaxies appear to contain only ~ 15% of the total mass.