4.5. Dark Matter Conferences 1975
The importance of dark matter for cosmological studies was evident, thus Tartu astronomers organised in January 1975 a conference in Tallinn devoted solely to dark matter. Historically this was the first conference on dark matter. This conference is not well known, so I give here the list of major talks:
Zeldovich: "Deuterium nucleosynthesis in the hot Universe and the density of matter";
Einasto: "Dynamical and morphological properties of galaxy systems";
Ozernoy: "The theory of galaxy formation";
Zasov: "The masses of spiral galaxies";
Fessenko: "Difficulties of the study of dynamics of galaxy systems";
Novikov: "The physical nature of galactic coronas";
Saar: "Properties of stellar halos";
Doroshkevich: "Problems of the origin of galaxies and galaxy systems";
Komberg: "Properties of the central regions of clusters of galaxies";
Vorontsov-Velyaminov: "New data on fragmenting galaxies".
As we see, the emphasis of the conference was on the discussion of the physical nature of dark matter and its role in the formation of galaxies. These preliminary studies demonstrated that both suggested models for coronas had difficulties. It is very difficult to explain the physical properties of the stellar corona, also no fast-moving stars as possible candidates for stellar coronas were found.
Stellar origin of dark matter in clusters was disussed by Napier & Guthrie (1975); they find that this is possible if the initial mass function of stars is strongly biased toward very low-mass stars. Thorstensen & Partridge (1975) discussed the suggestion made by Cameron & Truran (1971) that there may have been a pregalactic generation of stars (called now population III), all of them more massive than the Sun, which are now present as collapsed objects. They conclude that the total mass of this population is negligible, thus collapsed stars cannot make up the dark matter.
The gaseous corona of galaxies and clusters was discussed by Field (1972), Silk (1974), Tarter & Silk (1974) and Komberg & Novikov (1975). The general conclusion from these studies is that coronas of galaxies and clusters cannot consist of neutral gas (the intergalactic hot gas would ionise the coronal gas), but a corona consisting of ionised gas would be observable. Modern data show that part of the coronal matter in groups and clusters of galaxies consists of X-ray emitting hot gas, but the amount of this gas is not sufficient to explain flat rotation curves of galaxies.
The dark matter problem was discussed also during the Third European Astronomical Meeting in summer 1975. In contrast to the Tallinn Meeting now the major dispute was between the supporters of the dark matter concept and the older paradigm with conventional mass estimates of galaxies. The major arguments against the dark matter concept were summarised by Materne & Tammann (1976). They were as follows (see also Burbidge 1975):
The dark halo hypothesis is based on the assumption that companions are physical; if they are not then they do not measure the mass of the main galaxy, but characterise mean random velocities of galaxies;
Groups of galaxies are bound with conventional masses; the mean mass-to-luminosity ratios of groups are 4 and 30 for spiral and elliptical dominated groups, respectively;
The high masses of clusters may be explained by the high masses of the dominant cD galaxies; in other words - there is no extra mass in clusters;
Big Bang nucleosynthesis suggests a low-density Universe with the density parameter 0.05; the smoothness of the Hubble flow also favours a low-density Universe.
It was clear that by sole discussion only the presence and nature of dark matter cannot be solved, new data and more detailed studies were needed.