On the reality of compact groups of galaxies

The problem of the reality of Compact Groups of galaxies (CGs) is discussed. We separated two classes of groups: elongated, chain-like groups (I), and round ones (II). The number of chain-like groups among Hickson Compact Groups (HCGs) is about twice as large as that of the round ones. In Shakhbazian Compact galaxy Groups (ShCGs) the proportion of chain-like groups is smaller than that of the round ones. It is shown that some parameters of both classes of HCGs are quite different. The radial velocity dispersion (σv) is smaller in chain-like groups, and the interaction and merging processes are more efficient in them. The magnitude difference between the brightest and the second brightest galaxies in HCGs is larger in groups of class I, and is smaller in groups with larger number of members. The surface distribution in round CGs is concentrated towards the center. The above-mentioned differences allow us to conclude that CGs are real physical entities.