© CAMBRIDGE UNIVERSITY PRESS 1999

2.9. Velocity Dispersion

The typical radial (line-of-sight) velocity dispersion of galaxies in rich clusters (median value) is

(22)

The typical range of radial velocity dispersion in rich clusters (Struble and Rood 1991) is

(23)

A weak correlation between _r and richness exists; richer clusters exhibit, on average, larger velocity dispersion (Bahcall 1981). The observed velocity dispersion of galaxies in rich clusters is generally consistent with the velocity implied by the X-ray temperature of the hot intracluster gas (Section 3.5), as well as with the cluster velocity dispersion implied from observations of gravitational lensing by clusters (except possibly in the central core). Velocity dispersion and temperature profiles as a function of distance from the cluster center have been measured only for a small number of clusters so far. The profiles are typically isothermal [²_r(r) ~ T_x(r) ~ constant] for r 0.5-1h^-1 Mpc, and drop somewhat at larger distances.