Central cusp due to a super-massive black hole in axisymmetric models of elliptical galaxies

Abstract

We use numerical simulations to investigate the cusp at the centre of elliptical galaxies, due to the slow growth of a super-massive black hole. We study this problem for axisymmetric models of galaxies, with or without rotation. The numerical simulations are based on the `Perturbation Particles' method, and use GRAPEs to compute the force due to the cusp. We study how the density cusp is affected by the initial flattening of the model, as well as the role played by initial rotation. The logarithmic slope of the density cusp is found to be very much insensitive to flattening; as a consequence, we deduce that tangential velocity anisotropy -which supports the flattening- is also of little influence on the final cusp. We investigate via two different kinds of rotating models the efficiency with which a rotation velocity component builds within the cusp. A cusp in rotation develops only for models where a net rotation component is initially present at high energy levels. The eventual observation of a central rotational velocity peak in E galaxies has therefore some implications for the galaxy dynamical history.