4.6. Are Pairs of Galaxies Physical?
In mid 1970s the main arguments for the presence of dark halos (coronas) of galaxies and clusters of galaxies were statistical. In particular, the masses of double galaxies were determined by statistical methods. If companion galaxies used in mass determination are not real physical companions but random interlopers, then the mean velocity dispersion reflects random velocities of field galaxies and no conclusions on the mass distribution around giant galaxies can been made.
The difficulties connected with the statistical character of our arguments were discussed already during the Caucasus Winter School. Immediately after the school we started a study of properties of companion galaxies. The main question was: are companions true members of the satellite systems, which surround giant galaxies. Soon we discovered that companion galaxies are segregated morphologically: elliptical (non-gaseous) companions lie close to the primary galaxy whereas spiral and irregular (gaseous) companions of the same luminosity have larger distances from the primary galaxy; the distance of the segregation line from the primary galaxy depends on the luminosity of the primary galaxy (Einasto et al. 1974a). This result shows, first of all, that the companions are real members of these systems - random by-fliers cannot have such properties. Second, this result demonstrated that diffuse matter can have a certain role in the evolution of galaxy systems. The role of diffuse matter in galactic coronas was discussed in detail by Chernin, Einasto & Saar (1976). Morphological properties of companion galaxies can be explained, if we assume that at least part of the corona is gaseous. On the other hand, Komberg & Novikov (1975) demonstrated that coronas cannot be fully gaseous. Thus the nature of coronas remained unclear. Also we found that dynamical and morphological properties of primary galaxies are well correlated with properties of their companions (Einasto et al. 1976c). Brighter galaxies have companions which move with larger relative velocities than companions of fainter primaries. A further evidence of the large mass of the corona of our Galaxy came from the study of the dynamics of the Magellanic Stream (Einasto et al. 1976a).
The status of the dark matter problem in galaxies was discussed during the Commission 33 Meeting of the IAU General Assembly in Grenoble, 1976. Here arguments for the presence of dark halos and its non-stellar nature were again presented by Einasto, Jõeveer & Kaasik (1976b). But there remained two problems:
If the massive halo (or corona) is not stellar nor gaseous, of what stuff is it made of?
And a more general question: in Nature everything has its purpose. If 90 % of matter is dark, then this must have some purpose. What is the purpose of dark matter?