Dynamical Friction on Satellite Galaxies

For a rigid model satellite, Chandrasekhar's dynamical friction formula describes the orbital evolution quite accurately, when the Coulomb logarithm is chosen appropriately. However, it is not known if the orbital evolution of a real satellite with the internal degree of freedom can be described by the dynamical friction formula. We performed an N-body simulation of the orbital evolution of a self-consistent satellite galaxy within a self-consistent parent galaxy. We found that the orbital decay of the simulated satellite is significantly faster than an estimate from the dynamical friction formula. The main cause of this discrepancy is that the stars stripped out of the satellite are still close to the satellite, and increase the drag force on the satellite through two mechanisms. One is a direct drag force from particles in the trailing tidal arm, a non-axisymmetric force that slows the satellite down. The other is an indirect effect that is caused by the particles remaining close to the satellite after escaping. The force from them enhances the wake caused in the parent galaxy by dynamical friction, and this larger wake in turn slows the satellite down more than expected from the contribution of its bound mass. We found that these two have comparable effects, and that the combined effect can be as large as 20% of the total drag force on the satellite.