Galaxy Clusters

While Oort was carrying out his observations of stellar motions, Fritz Zwicky of Caltech discovered the presence of dark matter on a much larger scale through his studies of galactic clusters. A galactic cluster is an group of galaxies which are gravitationally bound. Our own galaxy, the Milky Way, is a member of a small cluster known as the Local Group. Using the same method employed by Oort, Zwicky determined the Doppler shifts of individual galaxies in one particular system, the Coma cluster--about 300 million light years away. Zwicky found nearly 10 times as much mass as observed in the form of visible light was needed to keep the individual galaxies within the cluster gravitationally bound. It was clear to Zwicky, as it had been to Oort, that a large sum of mass was extant which was simply not visible. At the time, astronomers referred to the material as "missing mass". However, this was deemed a misnomer as the mass was clearly present, but simply not visible. Hence, the more appropriate term "dark matter" came to supercede "missing mass". Since Zwicky's efforts, more recent measurements have found that certain galaxy clusters (and binary galaxies) have M/L ratios up to 300.

The mass-to-light ratio can also be evaluated by studying galaxy pairs, groups, and clusters. In each case, one measures velocities and length-scales, leading to a determination of the total mass required to provide the necessary self-gravity to stop the system from flying apart. The inferred ratio of mass-to-luminosity is about 100 in galaxy pairs, which typically have separations of about 100 kpc, and increases to 300 for groups and clusters of galaxies over a length scale of about 1 Mpc.