On the dynamics of the cores of galaxy clusters

We have determined the radial velocity-dispersion profiles (VDps) of 72 clusters, each with over 50 confirmed members. The velocity dispersion σv in clusters is found to be a strong function of radius; the largest gradients generally occur in the inner 0.5 h−1 Mpc. In 27 clusters the VDp is significantly centrally peaked, while in 11 cases we find an 'inverted' profile, i.e. σv increases with distance from the centre in the inner 0.5 h−1 Mpc. In four other clusters, whose VDps were unclassifiable, we find hints of a central inversion of σv. Several tests indicate that the observed fractions of peaked and inverted VDps cannot be explained as the result of noise in the data alone.