Slow evolution of elliptical galaxies induced by dynamical friction

Abstract

Many astrophysical problems, ranging from structure formation in cosmology to dynamics of elliptical galaxies, refer to slow processes of evolution of essentially collisionless self-gravitating systems. In order to determine the relevant quasi-equilibrium configuration at time t from given initial conditions, it is often argued that such slow evolution may be approximated in terms of adiabatic evolution, for the calculation of which efficient semi--analytical techniques are available. Here we focus on the slow process of evolution, induced by dynamical friction of a host stellar system on a minority component of "satellites", that we have investigated in a previous paper, to determine to what extent an adiabatic description might be applied. The study is realized by comparing directly N--body simulations of the stellar system evolution (in two significantly different models) from initial to final conditions in a controlled numerical environment. We demonstrate that for the examined process the adiabatic description is going to provide incorrect answers, not only quantitatively, but also qualitatively. The two classes of models considered exhibit generally similar trends in evolution, with one exception noted in relation to the evolution of the total density profile. This simple conclusion should be taken as a warning against the indiscriminate use of adiabatic growth prescriptions in studies of structure of galaxies.