Influence of the disc on the orbital decay of satellite galaxies

Using a formula derived from the Lynden-Bell-Kalnajs theory of angular momentum exchange, we study the angular momentum loss of a satellite galaxy which is moving on a nearly circular, co-planar orbit around a disc galaxy. Using this formula, estimates of the orbital decay time are computed for several different galaxy models. The results are then compared with the corresponding results from numerical simulations, and are found to be in agreement for the satellite sizes under study. The main difficulty in applying the decay formula is the fact that the decay time varies as the square of the satellite softening parameter, which is difficult to determine accurately. However, we find that it can be chosen in the same way in all cases we have studied. For our models, the dynamical friction of the disc is shown to be significantly larger than that of the halo.