Clusters of galaxies

A review is given of work on clusters of galaxies. During the last decade it was clearly established that the average mass-to-light ratio, (M/L), of galaxy clusters is much larger than that of the visible part of individual galaxies, which suggests the presence of missing mass in clusters. Flat outer rotation curves of individual spiral galaxies were discovered, which led to the hypothesis of missing mass in galaxies in the form of dark massive haloes with an R-2 density distribution. But the R-3 density distribution of observed halo tracers, the large range in values of M/L among systems of galaxies with similar galaxy content, and other difficulties with the missing mass hypothesis, admit the possibility that new dynamical physics may be required to resolve the M/L problem. Red-shift surveys for large homogeneous samples of galaxies have led to an improved knowledge of the three-dimensional distribution of galaxies in space. The basic new recognisable large-scale features are superclusters with characteristic sizes approximately 100 Mpc (330 million light years) and equally large galaxy-free voids. The infall velocity of the Local Group towards the Virgo cluster in the Local Supercluster has been measured. More extensive and improved data, as well as the new knowledge of superclustering of galaxies, have strengthened the assumption that galaxy red-shifts are caused by Doppler velocities.