Dynamical evolution of triplets of galaxies

By means of N-body simulations we study the global dynamics of triplets of galaxies, considering initial conditions (ICs) starting from 'maximum expan sion' and in virial equilibrium. Unlike previous studies we treat galaxies self-consistently, but we restrict ourselves to models with spherical symme try and do not consider the influence of a primordial common halo of dark m atter. Our results indicate that a low number of triple mergers is expected at the present epoch (approximate to 10 per cent) for collapsing triplets. Initia lly virialized conditions yield approximate to5 per cent of triple mergers in similar to 10 Gyr of evolution; hence, the three-galaxy problem has stab le states. No overmerging problem for these small groups of galaxies is fou nd. Their geometrical properties, as reflected by the Agekyan-Anosova map, do not show an excess of extreme hierarchical triplets. Unlike the three-bo dy problem no 'sling-shot' events are found during triple interactions, for both collapsing and virial ICs. The median velocity dispersion of observed compact triplets (sigma similar to 100 km s(-1)) is not well reproduced in our models at the present epoch: sigma similar to 50 km s(-1) for collapsi ng and sigma similar to 80 km s(-1) for virial. However, about 10 per cent of simulated triplets reaching the present epoch from maximum expansion hav e dynamical properties very similar to the median of Karachentev's compact triplets. Our median values agree, however, very well with the new data on triplets. We find that the median of the virial mass estimates does not ove restimate, in general, the mass of triplets, but underestimates it by appro ximate to 35 per cent. The median mass estimator appears as a somewhat bett er mass estimator. Analysis of the dynamical parameters, as well as information obtained from a pseudophase-plane constructed using their velocity dispersion and harmoni c radius, leads us to conclude that Karachentsev's compact triplets probabl y represent the most advanced stage of gravitational clustering of initiall y diffuse triplets. To test this thesis we suggest that triplets be studied within a cosmological scenario.