Dynamical friction in cuspy galaxies
  In this paper we treat the problem of the dynamical friction decay of a
massive object moving in an elliptical galaxy with a cuspidal inner
distribution of the mass density. We present results obtained by both
self-consistent, direct summation, N-body simulations, as well as by a new
semi-analytical treatment of dynamical friction valid in such cuspy central
regions of galaxies. A comparison of these results indicates that the proposed
semi-analytical approximation is the only reliable in cuspy galactic central
regions, where the standard Chandrasekhar's local approximation fails, and,
also, gives estimates of decay times that are correct at 1% respect to those
given by N-body simulations. The efficiency of dynamical friction in cuspy
galaxies is found definitively higher than in core galaxies, especially on more
radially elongated satellite orbits. As another relevant result, we find a
proportionality of the dynamical friction decay time to the -0.67 power of the
satellite mass, M, shallower than the standardly adopted 1/M dependence.
