Orbital decay of galactic satellites as a result of dynamical friction

We used N-body numerical simulations to investigate the orbital decay, caused by dynamical friction, of a satellite system (such as a globular cluster or a dwarf galaxy) which moves within a larger spherical stellar system (such as a galaxy). In our experiments the galaxies were simulated with mass points distributed in Plummer spheres with isotropic velocity distribution; those galaxies were self-consistent in some of our simulations, while in others the point masses moved in a rigid Plummer potential without interacting with each other. Satellites were represented with single rigid Plummer distributions that interacted with all the point masses making up the galaxies, and they moved along elongated orbits similar to those of galactic objects. We were particularly interested in low-mass satellites, and most of our investigation dealt with satellite masses of 0.005 and 0.01 of the galactic mass; to ascertain the limitations to the applicability of our results posed by the value of the satellite mass we also analysed several cases with larger masses (0.04 and 0.09). Our numerical results for low-mass satellites showed very good agreement with theoretical predictions obtained from a straightforward application of Chandrasekhar's dynamical friction equation, and allowed us to obtain approximate values of the Coulomb logarithm from the best possible fit between numerical and theoretical results. The structure of the galaxies is significantly altered during the orbital decay for satellite masses greater than 0.01, and we investigated those structural changes. Several experiments were done in order to determine the mechanism involved (local or global) in the satellite orbital decay. The results suggest that for the low mass satellites the orbital decay is mainly due to the dynamical friction, which is itself a local process, and that it is not affected by the global response induced in the galaxy.