F. Some Conclusions
From the inspection of widely separated interconnected galaxies we may formulate the following preliminary conclusions.
1. The interconnected galaxies are of all possible types including normal spirals, barred spirals, globular, elliptical and irregular systems. Any of these types may appear tied to a stellar system of the same structural type or to any of the other types.
2. Galaxies with conspicuous extended intergalactic connections seem to be generally absolutely bright or very bright.
3. Speculations as to the mode of formation of interconnected galaxies may follow two lines. A group may have been formed as a whole from a cloud of primordial gas and dust or it may be the result of a close encounter of galaxies already possessing their normal complements of stars, gases and dust. The structural appearance of the various formations shown in the Plates of this article speaks for the assumption that we here deal with the results of collisions of galaxies. This conclusion is supported by the observation that some of the relative differential radial velocities are so large that the member galaxies of some of our multiple systems are in the process of escaping from one another. As more spectra will be observed in the near future a final decision on this issue should soon be reached. It should be added that with the separations and the relative velocities observed the formations which we have discussed are ten to hundred million years old in order of magnitude.
4. From the structural features of the distribution of matter between widely separated galaxies as well as from the observed kinematic characteristics it is seen that as a result of encounters of galaxies matter is ejected into intergalactic space. Such encounters therefore constitute a direct source of supply for intergalactic matter, in the sense that this matter is not directly related to any specific stellar system but may exist freely until it is eventually swept up by galaxies passing through it. Also because of the observed violent disruptions of galaxies during close encounters dwarf stellar systems can be formed as it was suggested by the writer long ago (9, 13). Systems of this type, as they appear on the Plates VI and VIII and in Fig. 5 have in the meantime been found in great numbers in our extragalactic neighborhood. As a consequence of these discoveries drastic revisions had to be made with respect to the shape of the luminosity function of galaxies which according to the original investigators (2) had a maximum at an absolute photographic magnitude mp = - 14.2 but which from theory was predicted to increase monotonely with decreasing absolute brightness (13). As more and more data are being accumulated a luminosity function as it was foreseen by the theory has been emerging ever more distinctly (14, 16).
As a far reaching result from the new discoveries on intergalactic formations the former estimates for the average density of matter had to be revised and it now seems most likely that density is at least of the order of 10-26 grams/cm3. if HUBBLE'S old scale for the distances is used.
5. If the luminous filaments and bridges shown in the various Plates of this study are actually ejected during close encounters of galaxies, the "internal viscosity" of these galaxies must be considerably greater than is commonly assumed. Actually, through a quantitative study of oscillatory characteristics of stellar systems as well as of the dynamic interplay of gas and dust clouds with the stars imbedded in them, it can be shown that galaxies encountering each other will lead to the ejection of stars and of dispersed matter into intergalactic space. The occurrence of the taffy like filaments, however, still remains a puzzle which may be related to the still more general problem of the formation of almost stringlike spiral arms in many galaxies.
6. On collision of mechanical systems, internal oscillations within the various components are more readily excited than rotational motion. Indeed, in a head-on collision no moment of momentum is transmitted or generated, while oscillatory motions of more or less great amplitude are always set up. Strangely enough the analysts of the spectra of galaxies and of systems of galaxies have almost exclusively occupied themselves with the translational and rotational properties of nebulae. Oscillatory modes of motions of galaxies are virtually never mentioned in the astronomical literature. From the appearance of the interconnected multiple nebulae shown in this study it is nevertheless certain that oscillations within stellar systems must play a great role since it is obvious that along the filaments and bridges shown there exist differential motions directed essentially radially to or from the centers of the member galaxies involved.