Slow evolution of elliptical galaxies induced by dynamical friction - I. Capture of a system of satellites

The main goal of this paper is to set up a numerical simulation for the study of the slow evolution of the density and of the pressure tensor profiles of an otherwise collisionless stellar system, as a result of the interactions with a minority component of heavier particles. The effects that we would like to study are those attributed to slow collisional relaxation and generically called dynamical friction; in real cases, or in numerical simulations, the processes involved are complex, so that the relaxation associated with the granularity in phase space is generally mixed with and masked by evolution resulting from lack of equilibrium or from a variety of instabilities and collective processes. We start by revisiting the problem of the sinking of a satellite inside an initially isotropic, non-rotating, spherical galaxy, which we follow by means of N-body simulations using about one million particles. We then consider a quasi-spherical problem, in which the satellite is fragmented into a set of many smaller masses with a spherically symmetric initial density distribution. In a wide set of experiments, designed in order to bring out effects genuinely associated with dynamical friction, we are able to demonstrate the slow evolution of the density profile and the development of a tangentially biased pressure in the underlying stellar system, while we briefly address the issue of the circularization of orbits induced by dynamical friction on the population of fragments. The results of the simulations presented here and others planned for future investigations allow us to study the basic mechanisms of slow relaxation in stellar systems and thus may be of general interest for a variety of problems, especially in the cosmological context. Here our experiments are conceived with the specific goal to clarify some mechanisms that may play a role in the evolution of an elliptical galaxy as a result of the interaction between the stars and a significant population of globular clusters or of the merging of a large number of small satellites.