C. Recent Advances
A number of recent developments have shed important new light on the problems related to the structure and to the formation of multiple galaxies. Some of these developments are as follows.
1. Drs. M. L. HUMASON and N. U. MAYALL have now completed their catalogue of the redshifts in the spectra of over six hundred galaxies. Their data not only refer to the shifts in wave lengths but they also give a great wealth of information on the intrinsic character of the spectra of various types of individual and of multiple stellar systems.
2. With a view to the determination of the absolute characteristics of the components of multiple galaxies reliable knowledge on the absolute extragalactic distance scale is indispensable. In recent years data have come to light concerning the absolute luminosities of certain classes of variable stars which have made it necessary to abandon the old distance scale. Corrections will have to be applied which may change the distances to many nebulae and groups of nebulae by factors as high as four or five. Large uncertainties must also be kept in mind because of the recent discovery by the writer of obscuring intergalactic matter (16).
3. Sky surveys with the 18-inch and the 48-inch Schmidt telescopes on Palomar Mountain revealed the existence of a surprisingly large number of rather widely separated galaxies which appear connected by luminous intergalactic formations. Although the absolute distance scale may remain uncertain for some time to come the luminous bridges and filaments make it certain that two nebulae so connected are at the same distance, a fact which is of the utmost importance to astrophysical theory. It is estimated that with the 48-inch Schmidt several thousand pairs of widely separated galaxies have been found to be interconnected by faintly luminous formations. The author has only made a cursory search for close double and multiple nebulae as they appear on a few dozen plates taken with the 48-inch Schmidt. On the basis of this search it appears that probably several hundred thousand close doubles could be located with this instrument. A future thorough project along these lines will thus be rewarding.
4. The discovery of extended luminous intergalactic formations was not entirely accidental. Theoretical considerations had previously indicated that the internal viscosity of stellar systems is far greater than had been originally expected (9, 18). Because of this high viscosity it was concluded (19) that galaxies on close encounter will violently disrupt each other and much of the "debris" may be expected to escape into intergalactic space since the relative kinetic energy of many neighboring galaxies is often equal or even considerably greater than their internal gravitational potential energy. On the basis of these theoretical predictions the author therefore began an intensive search for intergalactic matter in the large clusters of nebulae, where encounters may be expected to occur most often. As a consequence of this search both luminous and dark internebular formations were found (15, 20). As an additional result of the theory the existence of many dwarf galaxies was forseen and observationally confirmed (19, 20).
5. As a consequence of the studies of widely separated inter-connected galaxies a reexamination of the structures of close pairs and multiples was started. Many of these were found to possess long extensions not previously known. New results were also obtained on the nature of the tidal actions in physical pairs and, with the help of the so-called analytical composite photography (21) a most promising exploration of the distribution of colours and of polarisation over the different parts of interacting stellar systems has been started. The statistics of the differences in apparent magnitudes of the member galaxies in multiple systems has confirmed the conclusion originally derived from theory (19) that the luminosity function of galaxies has no maximum but is monotonely increasing with decreasing brightness (14).
6. A study has been started of the apparent radial velocity differences of the members of interconnected galaxies. The magnitude of some of these differences (up to more than 7000 km./sec.) poses entirely new and unsuspected problems for astrophysical theory.
7. The advent of radio-astronomy begins to be of the greatest importance for the study of multiple galaxies. Indeed two galaxies lying in the same line of sight might be mistaken for a physical double although actually widely separated. If however the objects in question acts as a radio source one can be fairly certain that he deals with a physical double although the apparent radial velocities of the two components may be quite different, as it is the case for NGC 1275, to be discussed later.
Considering all of the new aspects mentioned in the preceding as well as the rapid developments now taking place, it would be clearly premature to attempt a systematic classification and a basic analysis of the properties of multiple galaxies. It will be more satisfactory for the present to retrace the historical development as it was experienced by the writer and to discuss a number of systems which possess particularly characteristic features and for which significant data have been secured. As mentioned before, the whole recent developments were started as an attempt to prove certain phases of astrophysical theory and they led almost immediately to the discovery of certain multiple galaxies whose members are connected by extended luminous intergalactic formations.