Non Locality of Collective Effects Related to Dynamical Friction in Elliptical Galaxies

To a large extent, the basic mechanism of dynamical friction remains an open problem. In fact, the classical idealized local description, given by Chandrasekhar, is known to be subject to a number of limitations, possibly related to the global nature of the collective processes that are involved. Here we present a study of the properties of dynamical friction in galaxy models, derived from a realistic distribution function, that are characterized by significant density concentration and by pressure anisotropy biased in the radial direction. The problem is addressed by means of dedicated N-body simulations. By following the fall of heavy objects inside the host galaxy, we find that the classical theory is better suited to describe dynamical friction only outside cores of models with low concentration, but fails in more concentrated models, more appropriate to describe elliptical galaxies. In turn, pressure anisotropy in the host galaxy does not appear to have significant effects.