Annu. Rev. Astron. Astrophys. 2002. 40: 539-577 Copyright © 2002 by Annual Reviews. All rights reserved

2. PHYSICAL PROPERTIES OF GALAXY CLUSTERS

Clusters of galaxies were first identified as large concentrations in the projected galaxy distribution (Abell 1958, Zwicky et al. 1966, Abell et al. 1989), containing hundreds to thousands galaxies, over a region of the order of ~ 1 Mpc. The first observations showed that such structures are associated with deep gravitational potential wells, containing galaxies with a typical velocity dispersion along the line-of-sight of _v ~ 10³ km s^-1. The crossing time for a cluster of size R can be defined as

(1)

Therefore, in a Hubble time, t_H 10 h^-1 Gyr, such a system has enough time in its internal region, 1 h^-1 Mpc, to dynamically relax - a condition that can not be attained in the surrounding, ~ 10 Mpc, environment. Assuming virial equilibrium, the typical cluster mass is

(2)

Smith (1936) first noticed in his study of the Virgo cluster that the mass implied by cluster galaxy motions was largely exceeding that associated with the optical light component. This was confirmed by Zwicky (1937), and was the first evidence of the presence of dark matter.