DYNAMICAL FRICTION IN DISC GALAXIES WITH NONZERO VELOCITY DISPERSION

We investigate the force of dynamical friction on a softened point mas s orbiting inside a disc galaxy on a planar circular orbit. The effect s of velocity dispersion in the disc on the force are studied both the oretically and by means of numerical simulations. The analytical resul ts, valid for small values of the velocity dispersion, show that the t otal effect of the velocity dispersion is relatively small, although t he contributions from different resonant terms change by large amounts . For non-self-gravitating simulations with small disc masses our anal ytical predictions are in acceptable agreement with the measured force . For larger values of the velocity dispersions the frictional force i n non-self-gravitating models is still only weakly affected for the pe rturber masses that we consider. We also find that the inclusion of se lf-gravity in the simulations has only a moderate effect on the force of dynamical friction. As a summary of our results we propose a simple formula which under most practical circumstances should give a suffic iently accurate estimate for the dynamical friction from a disc.