4.4. Galactic Coronas

In spring 1972 I was asked to give an invited review on Galactic models at the First European Astronomy Meeting in Athens. At this time population models of galaxies had been calculated already for 5 galaxies of the Local Group and the giant elliptical galaxy M87 in the Virgo cluster. New rotation velocities suggested the presence of almost flat rotation curves on the periphery of galaxies, thus it was increasingly difficult to accept the previous concept of large non-circular motions. On the other hand, recently finished calculations of the physical evolution of stellar populations confirmed our previous view that it is extremely difficult to accept a stellar origin of the hypothetical population. My collaborator Enn Saar suggested to abandon the idea that only stellar populations exist in galaxies, to accept an idea that there may exist a population of unknown nature and origin and to look which properties it should have using available data on known stellar populations. Quickly a second set of models for galaxies was calculated, and parameters for the new dark population were found. To avoid confusion with the conventional halo population I suggested to call the new population "corona" (Einasto 1974b). The available data were insufficient to determine the outer radii and masses of coronas. Rough estimates indicated that in some galaxies the mass and radius of the corona may exceed considerably the mass and radius of all stellar populations, taken together.

To determine the parameters of galactic coronas more accurately distant test bodies are needed. After some period of thinking I realised how it is possible to check the presence of dark coronas around galaxies. If coronas are large enough, then in pairs of galaxies the companion galaxies move inside the corona, and their relative velocities can be used instead of galaxy rotation velocities to find the distribution of mass around giant galaxies. This test showed that the radii and masses of galactic coronas exceeded the radii and masses of parent galaxies by an order of magnitude! Together with A. Kaasik and E. Saar we calculated new models of galaxies including dark coronas.

In those years Soviet astronomers had the tradition to attend Caucasus Winter Schools. Our results of galactic mass modelling were reported in a Winter School in 1972. The next School was hold near the Elbrus mountain in a winter resort, in January 1974. The bottom line of my report was: all giant galaxies have massive coronas, therefore dark matter must be the dominating component in the whole universe (at least 90 % of all matter). In the Winter School prominent Soviet astrophysicists as Zeldovich, Shklovsky, Novikov and others participated. In the discussion after the talk two questions dominated: What is the physical nature of the dark matter? and What is its role in the evolution of the Universe? A detailed report of this study was sent to "Nature" (Einasto, Kaasik & Saar 1974).

The need for massive halos had been already suggested by Ostriker and Peebles (1973) to stabilise galaxies against bar formation. Soon after our "Nature" paper Ostriker, Peebles and Yahil (1974) published similar results using similar arguments. They used the conventional term "halo" for the dark population apparently not realising that this population cannot be of stellar origin.