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 expansion’ and in virial equilibrium. Unlike previous studies we treat galaxies self‐consistently, but we restrict ourselves to models with spherical symmetry and do not consider the influence of a primordial common halo of dark matter. Our results indicate that a low number of triple mergers is expected at the present epoch (≈10 per cent) for collapsing triplets. Initially virialized conditions yield ≈5 per cent of triple mergers in ∼10 Gyr of evolution; hence, the three‐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. Unlike the three‐body problem no ‘sling‐shot’ events are found during triple interactions, for both collapsing and virial ICs. The median velocity dispersion of observed compact triplets (σ∼100 km s −1 ) is not well reproduced in our models at the present epoch: σ∼50 km s −1 for collapsing and σ∼80 km s −1 for virial. However, about 10 per cent of simulated triplets reaching the present epoch from maximum expansion have 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 overestimate, in general, the mass of triplets, but underestimates it by ≈35 per cent. The median mass estimator appears as a somewhat better mass estimator. Analysis of the dynamical parameters, as well as information obtained from a pseudo‐phase‐plane constructed using their velocity dispersion and harmonic radius, leads us to conclude that Karachentsev's compact triplets probably represent the most advanced stage of gravitational clustering of initially diffuse triplets. To test this thesis we suggest that triplets be studied within a cosmological scenario.