Observational Properties of Compact Groups of Galaxies

Abstract

Compact groups are small, relatively isolated, systems of galaxies with projected separations comparable to the diameters of the galaxies themselves. Two well-known examples are Stephan’s Quintet (Stephan, 1877) and Seyfert’s Sextet (Seyfert 1948a,b). In groups such as these, the apparent space density of galaxies approaches 106 Mpc-3, denser even than the cores of rich clusters. The apparent unlikeliness of the chance occurance of such tight groupings lead Ambartsumyan (1958, 1975) to conclude that compact groups must be physically dense systems. This view is supported by clear signs of galaxy interactions that are seen in many groups. Spectroscopic observations reveal that typical relative velocities of galaxies in the groups are comparable to their internal stellar velocities. This should be conducive to strong gravitational interactions – more so than in rich clusters, where galaxy velocities are typically much higher. This suggests that compact groups could be excellent “laboratories” in which to study galaxy interactions and their effects.