Merger Rate of Equal‐Mass Spherical Galaxies

We present cross sections and reaction rates for merging to occur during encounters of equal-mass spherical galaxies. As an application, we determine the rate of galaxy merging in clusters of galaxies. We present results for two types of Plummer models (a full and a truncated one), two King models, and the Hernquist model. Cross sections are determined on the basis of a large number (D500) of simulations of galaxy encounters, using the 10 Gigaops GRAPE-3A special purpose computer. We characterize the overall merger rate of galaxies in a galaxy cluster by a single number, derived from our cross sections by an integration over galaxy encounter velocities in the limit of a constant density in velocity space. For small clusters, where the cluster velocity dispersion may not signicantly exceed the internal velocity dispersion of the individual galaxies, this constant-density approximation may not be valid. For those cases, we present separate results, based on integrations of our cross sections over Maxwellian velocity distributions. Finally, tidal e ects from the cluster potential, as well as from neighboring galaxies, may prevent a barely bound galaxy pair from spiraling in after their rst encounter. We give a quantitative estimate of the resulting reduction in the actual merger rate that is due to these tidal interactions.