The galactic groups associated with M101 and M81, for instance, do not actually contain a single elliptical galaxy; they consist only of spiral and irregular galaxies. The galactic group in Sculptor (studied by de Vaucouleurs) contains only galaxies of the Sc type and irregular galaxies. Finally, the Local Group does not contain any elliptical galaxies of high luminosity either, but it does include some elliptical galaxies of low and medium luminosity.
It is interesting to note too that the Local Group essentially consists of two quite small galactic groups, with dimensions close to those of multiple galaxies. The first group comprises our Galaxy, the Large and Small Magellanic Clouds, and apparently a few galaxies of the type of the system in Sculptor as well. The second group contains the Andromeda Nebula with its four companions and M33. However, this division can be considered valid only for galaxies of high and medium luminosity. Dwarf galaxies, on the other hand, may quite possibly be distributed continuously throughout the entire Local Group. It should be added that the total mass of the Local Group is mainly determined by the masses of the two galaxies which are essentially the centers of the above-mentioned two subgroups (M31 and our Galaxy).
In some cases, galactic clusters which have many members occur in twos and threes, and thus form multiple galactic clusters. It was mentioned above that galaxies as a rule represent closed stellar systems. There are cases, however, when the systems are not isolated, and three categories of such objects will now be described.
_ a) Interacting galaxies. These are cases in which two galaxies are close together, and the presence of one galaxy has a strong effect on the structure of the other. Many examples of interacting galaxies are listed in the atlas of B. A. Vorontsov-Vel'yaminov, who has contributed considerably to the study of these interesting objects. The observed interactions may be explained in two ways: 1) as a result of tidal action, and 2) as a result of the separation of two galaxies which were initially together. In the latter case the "interactions" are considered to be a consequence of the separation process.
b) Pairs of galaxies connected by bridges or bars.
Many instances of this have been reported by Zwicky, who has demonstra ted that the bars are made up of stars. Jets, or streamers, emanating from the central regions of some spherical galaxies, and containing blue condensations made up of dwarf galaxies, border upon the bars. * It has been noted that the streamer joining a large galaxy to a dwarf galaxy also resembles a bar, and in such cases there is little doubt but that the dwarf galaxy detached itself from the central core of the main galaxy. It thus seems more plausible to assume that bridges and bars are in fact the result of a division of a single galaxy into two parts.
c) Radio galaxies. Previously, radio galaxies were assumed to be the result of random collisions between two independent stellar systems, and the energy of the observed radio emission was assumed to come from the collision of the gaseous masses contained in the two galaxies. However, the facts have not supported such a hypothesis. All the data tend to indicate that radio galaxies represent some (possibly very brief) stage in the internal evolution of galaxies of very high luminosity (supergiant galaxies).
The radio-emission intensity of a galaxy is apparently closely connected with the genesis of new intragalactic formations, such as condensations and streamers (which are expelled from the center), spiral arms, and even whole new galaxies. In other words, in some cases a galactic center divides, and a new galaxy forms in the depths of the old one. This is why radio galaxies are often extremely compact systems consisting of an old galaxy and new formations, the latter usually being still embedded in the parent galaxy.
It should be noted that all the types of nonisolated galaxies described above together constitute only a small percentage of the total number of galaxies. There is good reason to believe that such cases of nonisolation occur at a definite stage of galactic evolution (specifically, when new galaxies are formed).
Despite the significant advances which have been made in the study of the spatial distribution of galaxies, there are still many important problems which remain unsolved. Let us now consider some of these.