3.6. Velocity Dispersions in Clusters
Radial-velocity observations have been made in a number of clusters, especially to obtain data for cosmological tests. Usually, however, the mean radial velocity of a cluster is estimated from redshift measures of only two or three individual members - too few to provide much information about the internal kinematics of the cluster. For a few clusters and groups, however, there are enough data to allow a more or less meaningful estimate of the dispersion of velocities to be made. Data for several such clusters are given in table 6. Successive columns list the name of the cluster or the Abell catalog number (or both), the mean radial velocity (corrected for galactic rotation), the square root of the dispersion in radial velocities, the number of galaxies whose measured redshifts were used in calculating these quantities, and the authority for the observations.
|Cluster||<Vr> (km s-1)||<vr2>1/2 (km s-1)||n||References|
|(around NGC 3627)||787||260||18||1, 2|
|( 3½h; -36°)||1452||287||12||1, 2|
|Pegasus I cluster|
|(around NGC 7619)||3836||260||6||1|
|Group around NGC 383||5274||504||9||1|
|(around NGC 541)||5321||406||41||4|
|(around NGC 6166)||8736||541||15||6|
REFERENCES. - (1) Humason, Mayall, and Sandage 1956; (2) Mayall and de Vaucouleurs 1962; (3) de Vaucouleurs 1961b; (4) Zwicky and Humason 1964a (they do not state whether or not their radial velocities are corrected for galactic rotation); (5) Lovasich et al. 1961; (6) Minkowski 1961; (7) Burbidge and Burbidge 1959b.
Three points should be noted: (1) The square roots of the dispersion given are in radial velocities (i.e., as seen from the galactic center). If the velocity field throughout a cluster is isotropic, the true value should be 3 times the value given. If, on the other hand, the motions of member galaxies are largely radial in a cluster, the true square root of the dispersion should be less than 3 times the tabulated value because most of the galaxies measured are near the projected center of a cluster where a majority would be expected to be moving nearly in the line of sight. (2) Redshifts can be measured only for the brightest galaxies in most clusters. If a degree of statistical equilibrium exists in a cluster, the fainter galaxies should have a larger dispersion in velocity; if not, the tabulated values may be near the true ones, except for projection effects. (3) Typical individual measures of redshifts of galaxies carry probable errors of from 50 to 200 km s-1. When only a few galaxies in a cluster are measured, the velocity dispersion can be considerably in error for this observational reason alone.
For a more current and complete compilation, see Noonan (1973).