Dynamical evolution of triplets of galaxies

Abstract

By means of N-body simulations we study the global dynamics of triplets of galaxies, considering initial conditions starting from `maximum expansion' and in virial equilibrium. Unlike previous studies we treat galaxies self-consistently. We do not consider the influence of a primordial common halo of dark matter. Our results indicate that a low number of triple mergers are expected at the present epoch (~10%) for collapsing triplets. Initially virialised conditions yield ~5% of triple mergers in ~10 Gyr of evolution; hence, the 3-galaxy problem has stable states. No overmerging problem for these small groups of galaxies is found. Their geometrical properties, as reflected by the Agekyan-Anosova map, do not show an excess of extreme hierarchical triplets. The median velocity dispersion of observed compact triplets (~100 km/s) is not well reproduced in our models at the present epoch. However, about 10% of simulated triplets starting from maximum expansion have dynamical properties very similar to them. Our median values agree, however, very well with new data on triples. We find that the median mass estimators performs somewhat better than the virial mass estimate. The VME underestimates mass by \~35%. We conclude that Karachentsev's compact triplets probably represent the most advanced stage of gravitational clustering of initially diffuse triplets.