Decay of galaxy satellite orbits by dynamical friction

Abstract

The decay of the orbit of a satellite galaxy around a large galaxy is investigated using a three-dimensional model in which the large galaxy is represented by a self-consistent 5000-particle distribution (polytrope of index 3). The satellite is modelled with a softened point mass. We find that the global responses in the large galaxy which are induced by the presence of the satellite are not dynamically important. The decay of the satellite orbit can be well described by Chandrasekhar's formula for dynamical friction, provided that variations in density and corrections for streaming motions are taken into account. These results imply that for the systems discussed in this paper the decay process through dynamical friction can be characterized as a local process. For a variety of satellite masses and radii, we determine the coefficient of dynamical friction ln Λ at many instants during the decay of the orbit, and find it to be nearly constant, except in the outer tenth of the main galaxy's mass distribution.