At any rate, the case of NGC5128 (Centaurus A) also shows that galactic nuclei are capable of ejecting either tremendous clouds of relativistic electrons or else material which can subsequently produce such clouds.
In any case, then, radio galaxies are systems in which the central nuclei show signs of intense activity ultimately leading to the formation of new condensations, new subsystems, and possibly even new galaxies. We may therefore safely say that in these cases the nuclei are c o s m o g o n i c ally active , although we still do not know where the masses involved in this activity come from.
E) There are some giant galaxies which have streamers coming out of their central regions. These streamers contain blue galaxies with absolute magnitudes of about -15 (that is, with luminosities higher than that of the condensation in NGC4486). Typical examples of such galaxies are NGC3561 and IC1182. The ejection of such condensations is another type of cosmogonic activity in galactic nuclei.
F) The fact that the spiral arms originate from the galactic nuclei themselves suggests that the generation of the arms is also directly associated with the nucleus.
G) Radio observations of the center of our Galaxy, carried out by Pariiskii and others, indicate that the state of the nucleus, which presumably consists mostly of new stars, differs markedly from the states of other aggregates of such stars (for instance, the globular clusters). The galactic nucleus itself is a source of thermal radio emission, while the surrounding region (with a diameter of about 500 parsecs) is a region of strong nonthermal radiation. These facts imply that the physical states of galactic nuclei differ greatly from the states of ordinary stellar groupings.
The quantitative evaluation of the ejected mass is one of the most important unsolved problems related to ejections and the efflux of matter from galactic nuclei. This applies equally well to galaxies whose central regions produce emission lines, and to radio galaxies and other cases involving discrete ejections. Even the meager information now available indicates that the findings may prove to be incompatible with the law of conservation of energy (and mass) in its present form, so that a generali zation of this law may be necessary.
We have seen that the most important processes taking place in large galaxies are determined by the activity of the galactic nuclei. This activity can take various forms, and these were discussed above. There are two types of nuclear activity which are especially interesting; one of these is related to the formation of spiral arms and the other is related to the formation of the stars and star clusters of the spherical component [Population II stars]. These processes apparently occur during different evolutionary stages and are accompanied by corresponding changes in the nuclei. It should be noted too that the different types of subsystems must be produced by different kinds of processes. For example, galaxy M32 apparently does not contain any globular clusters, whereas the other companion of the Andromeda Nebula (NGC205) contains at least nine of them. The most surprising thing with respect to this is that globular clusters are present in galaxies with very low density gradients.
If we adopt the hypothesis that galaxies are formed from diffuse star clouds, it is reasonable to assume that dense formations like globular clusters will appear in systems containing regions of very high density (and a high density gradient). Of course, such qualitative considerations cannot be considered satisfactory. The significant thing, however, is that the number of globular clusters per unit luminosity of Population II stars varies from system to system.
We thus obtain an additional parameter for the description of spherical systems and subsystems. The relationship between this parameter and the other parameters of these systems (total luminosity and density gradient) must be determined by observation.
Statistical data pertaining to multiple galaxies and galactic clusters show that these systems could not have been formed by the mutual capture of initially independent galaxies. The components of these systems must therefore have a common origin. This point was considered in detail in our report at the Solvay Conference in 1958.