Evolution of Compact Groups of Galaxies I . Merging rates

We discuss the merging rates in compact groups of 5 identical elliptical galaxies. All groups have the same mass and binding energy. We consider both cases with individual halos and cases where the halo is common to all galaxies and enveloping the whole group. In the latter situation the merging rate is slower if the halo is more massive. The mass of individual halos has little influence on the merging rates, due to the fact that all galaxies in our simulations have the same mass, and so the more extended ones have a smaller velocity dispersion. Groups with individual halos merge faster than groups with common halos if the configuration is centrally concentrated, like a King distribution of index Ψ = 10. On the other hand for less concentrated configurations the merging is initially faster for individual halo cases, and slower after part of the group has merged. In cases with common halo, centrally concentrated configurations merge faster for high halo-to-total mass ratios and slower for low halo-to-total mass ratios. Groups whose virial ratio is initially less than one merge faster, while groups that have initially cylindrical rotation merge slower than groups starting in virial equilibrium. In order to test how long a virialised group can survive before merging we followed the evolution of a group with a high halo-to-total mass ratio and a density distribution with very little central concentration. We find that the first merging occurred only after a large number of crossing times. A reasonable calibration of our computer units shows that this time should be larger than a Hubble time. Therefore, our simulations suggest that, at least for appropriate initial conditions, the longevity of compact groups is not necessarily a problem, thus presenting an alternative explanation to why we observe so many compact groups despite the fact that their lifetimes seem short.