TRANSIENT PHENOMENA IN GALAXIES Up to this point we have considered galaxies as static formations. However, transient phenomena also occur in galaxies, especially in the supergiants, and these phenomena are of great interest.
We are not referring here to the processes of stellar formation which take place in 0 and T associations, although these play a significant role in galactic evolution. What we have in mind are more rapid changes which are directly observable. It is noteworthy that most of these transient phenomena are associated with galactic nuclei and that they may even be regarded as manifestations of activity of the nuclei.
A) A stream of neutral hydrogen is flowing out of the central region of our Galaxy. This phenomenon was discovered by Dutch astronomers as a result of observations of the 21-cm radio-emission line of hydrogen. The same sort of gas-efflux process was detected for the nucleus of M31 by Munch, as a result of studies of the X= 3727 A line. In both cases the mass of the efflux amounts to about 1 solar mass per year. Oddly enough, this figure does not agree with existing estimates of the masses of galactic nuclei (about solar masses).
B) Seyfert has demonstrated that for some galaxies with high-luminosity nuclei the X = 3727 A emission line is broadened considerably, which corresponds to velocities of motion of the order of several thousand kilo meters per second. Such velocities exceed the usual escape velocities of galaxies. This undoubtedly indicates that intense fluxes of matter are ejected from the nuclei at high velocities and then dispersed throughout space. The amount of ejected matter in this case is apparently much greater than that released in our Galaxy and in M31. Presumably, the blue galaxies in Ara, which have strong emission lines in the regions near the nuclei, are of a similar nature.
C) The X=3727 A line is also observed at the very center of radio galaxy NGC4486, and it is apparently associated with a quite intensive gas efflux at a velocity of about 500 km/sec. If this is correlated with the fact that a radial jet issues from the center of the galaxy and contains condensations which are strongly emitting in the radio range, we can conclude that the condensations were expelled from the central nucleus of the galaxy at high velocities. The light of these condensations is
polarized, indicating that they contain high-energy electrons. These formations, however, are not on the same scale as the Crab Nebula, since the energy of their radio emission (measured in absolute units) is about ten million times higher. If we take into account that in this case the radio emission lasts at least a thousand times longer as well, then it follows that the energy reserves in these condensations are a billion times greater than the total energy reserves of the Crab Nebula. This means that, in terms of energy and mass, these condensations are objects which correspond to small galaxies, and this is found to agree with their absolute photographic magnitudes.
It is not yet known whether these condensations are ejected from the galactic nucleus as already-formed clouds of relativistic electrons, or (which is more probable) whether the nucleus expels objects which in turn continually produce new streams of such electrons. What is important, however, is that the nucleus of a giant galaxy is able to eject such immense condensations,. a fact which does not seem to agree with the information we have on the masses of galactic nuclei.
D) It is much more difficult to explain the phenomena occurring in other radio galaxies. It is known, though, that galaxy NGC1275 (Perseus A) is one of the Seyfert galaxies, in which the X = 3727 A line is broadened considerably for the central region. Thus, in this case also matter is ejected from the nucleus at a high rate.
The presence of two nuclei in radio galaxy Cygnus A may be the result of a recent splitting of one parent nucleus. According to our previous considerations, such a division should lead to the formation of subsystems with different centers and subsequently to the formation of a binary galaxy.