N-body simulations with perturbation particles - II. Dynamical friction at a distance

The time-scale of decay by dynamical friction for interacting galaxies is quite short, of the order of one orbital period, as soon as the systems interpenetrate. The global merging time-scale between galaxies is thus determined by the weaker friction occurring when galaxies are distant. It is difficult to study friction at a distance by conventional N-body simulations, since the weak perturbation is buried in particle noise. We apply here the technique of `perturbation particles' to investigate the distant friction between two elliptical galaxies. We rediscuss the scaling of merging times with the initial separation between galaxies, taking into account previous numerical simulations or extrapolations of the Chandresekhar formulation.