Dynamical Evolution of Clusters Of Galaxies, II

Abstract

Earlier numerical work on N -body systems has been continued by studying more advanced stages of dynamical evolution at higher accuracies. New features of data analysis include different methods for computing relaxation time and a consistent elimination of distant escapers from bound clusters. The overall relaxation time is in good agreement with that predicted by Chandrasekhar theory for centrally concentrated clusters and shows little variation with time, whereas the mean relaxation time for each mass group is well correlated with a pronounced mass segregation, the heavy members forming a dense nucleus. A stable nucleus is also maintained in cases where a cosmological expansion effect is included and more than half the members are lost to the field. There is no approach to equipartition of kinetic energy for the clusters as a whole and even in the central regions the equipartition factor is small compared to the mass ratio. The expected predominance of elongated orbits in the outer cluster regions is confirmed. At the same time the total velocity distribution develops an excess at high and low velocities as compared to a Maxwellian. Escape rates for different masses have been derived using the computed relaxation times. There is some evidence for selective escape among light members in isolated clusters and the total mass loss appears to be greater than in the case of an equal mass distribution. The dispersion of binding energy increases with time and may be used to denote the dynamical age of simulated clusters.