Size of the Coma cluster

STUDIES of counts of galaxies in large regions centred on the Coma cluster led Zwicky to conclude that the radius of the Coma cluster is at least 6° (13 Mpc for H = 55 km s −1 Mpc −1 ) 1 . Outside this radius, the contribution to the counts from foreground and background galaxies overwhelms the contribution from the cluster, rendering it invisible. The technology of image intensifier tubes has now advanced to such an extent that it is feasible to obtain line of sight (radial) velocities of large numbers of galaxies in fields of a cluster. These velocities can be used to distinguish cluster members from foreground and background galaxies 2 . Such studies have confirmed that the Coma cluster extends to at least 5° (ref. 3).