4.7. Additional Evidence for Dark Halos
In mid 1970s Vera Rubin and her collaborators developed new sensitive detectors to measure rotation curves of galaxies at very large galactocentric distances. Their results suggested that practically all spiral galaxies have extended flat rotation curves (Rubin, Ford & Thonnard 1978, 1980, see also a review by Rubin 1987). Now, for the first time, it was possible to determine the mass distribution in individual galaxies out to distances far superior to previous data. The internal mass of galaxies rised with distance almost linearly up to the last measured point (see Fig. 6 of Rubin et al. 1978). The concept of the presence of dark matter halos around galaxies was confirmed with a high confidence.
Another very important measurement was made by Faber et al. (1977). They measured the rotation velocity of the Sombrero galaxy, a S0 galaxy with a massive bulge and a very weak population of young stars and gas clouds just outside the main body of the bulge. Their data yielded for the bulge a mass-to-luminosity ratio M / L = 3, thus confirming our previous estimates based on less accurate data, and calculations of the physical evolution of galaxies. Velocity dispersion measurements of high accuracy also confirmed lower values of mass-to-luminosity ratios of elliptical galaxies (Faber & Jackson 1976). These results showed that the mass-to-luminosity ratios of stellar populations in spiral and elliptical galaxies are similar for a given colour (the assumption used in our model calculations), and the ratios are much lower than accepted in most earlier studies.
More recently the masses of clusters of galaxies have been determined using the temperature of hot X-ray emission gas in clusters, and by gravitational lensing. These data are discussed in other reports during this School.
By the end of 1970s most objections against the dark matter hypothesis were rejected. In particular, luminous populations of galaxies have found to have lower mass-to-luminosity ratio than expected previously, thus the presence of extra dark matter both in galaxies and clusters has been confirmed. However, the nature of dark matter and its purpose was not yet clear. Also it was not clear how to explain the Big Bang nucleosynthesis constraint on the low density of matter, and the smoothness of the Hubble flow.