The above-mentioned data on the ejection from galactic nuclei of condensed masses which turn into complete galaxies of medium or low luminosity, and also the data on the division of galactic nuclei, indicate that multiple systems and whole groups are probably created as a result of the division of a single parent nucleus into several nuclei. This division may take place in successive stages.
In cases when a group contains a central galaxy with a high luminosity, the formation of faint galaxies must be connected primarily with activity of the nucleus of the high-luminosity galaxy.
The nuclei of giant and supergiant galaxies have very high activities, as indicated by the fact that radio galaxies are usually among the brightest members of the clusters of which they form a part. If one of the galaxies in such a cluster is clearly predominant, this usually turns out to be the radio galaxy itself.
Observations show that although all the large clusters contain supergiant galaxies, only a small number of these are radio galaxies. Radio emissive activity must accordingly constitute a relatively brief phase in the evolutionary history of a galaxy. The liberation of radio-emissive agents is presumably a phenomenon associated with the ejection of very large masses from the galactic nuclei, and it probably occurs only at a certain stage of a given cosmogonic process.
Even though extragalactic astronomy has at its disposal considerable means for studying the activity of galactic nuclei, the information we have on the various forms of this activity is still extremely meager. We know even less about the parameters characterizing the integral properties of the nuclei (luminosity, mass, color, size, and rotation). Finally, we know nothing about the internal structure of galactic nuclei. Consequently, this particular aspect of extragalactic astronomy deserves extensive study. Let us now list some of the tasks involved in such a study.
1. Determination of whether all galaxies have nuclei. If some do not, then the characteristics of these must also be determined.
2. Determination of the integral characteristics of the nuclei for as many galaxies as possible. With respect to this, the difficulty involved in
studying galaxies with high density gradients must be taken into account. It should be noted, though, that many galaxies of type Sc have nuclei which are so distinct that they can be studied without much interference from the central condensation around the nucleus.
3. Determination of the relationship between the integral parameters of galactic nuclei and the parameters of galaxies.
4. The study of the spectra of nuclei, in order to detect emission lines, rotation, and the ejection of matter.
5. The study of the relation between the nucleus and the bar in a barred galaxy, and also of the relation between the bar and the efflux of matter from the nucleus.
6. The study of galaxies with multiple nuclei, and of the radial velocities of the individual components of these nuclei.
7. Determination of the number of globular clusters as a function of the nature of the galactic nucleus.
In the foregoing we have presented some considerations of cosmogonical nature relating to the origin of galaxies. In all cases, it was attempted to remain within the domain of facts and not to make speculations. An analysis of the observations shows that the phenomena related to galactic origins are so unusual that it would have been impossible to predict them on the basis of any theoretical considerations. Thus, we are here confronted by a striking phenomenon which has recurred continually throughout the history of science. Whenever a new domain of phenomena is investigated, unsuspected relationships are discovered, which go beyond the framework of existing concepts. This, of course, makes each such domain of phenomena all the more interesting. Consequently, we must gather facts and make observations even more carefully, since more precise data on the actual objects and more extensive information on the structures of the various parts of galaxies, together with a thorough analysis of the findings, are the only means of solving the difficult problems related to this subject.