Satellite decay in flattened dark matter haloes

We carry out a set of self-consistent N -body calculations to compare the decay rates of satellite dwarf galaxies orbiting a disc galaxy embedded in a dark matter halo (DMH). We consider both spherical and oblate axisymmetric DMHs of aspect ratio q (h) =0.6. The satellites are given different initial orbital inclinations, orbital periods and mass. The live flattened DMHs with embedded discs and bulges are set-up using a new fast algorithm due to Boily, Kroupa & Penarrubia called magalie. We find that the range of survival times of satellites within a flattened DMH becomes similar to100 per cent larger than the same satellites within a spherical DMH. In the oblate DMH, satellites on polar orbits have the longest survival time, whereas satellites on coplanar prograde orbits are destroyed most rapidly. The orbital plane of a satellite tilts as a result of anisotropic dynamical friction, causing the satellite's orbit to align with the plane of symmetry of the DMH. Polar orbits are not subjected to alignment. Therefore the decay of a satellite in an axisymmetric DMH may provide a natural explanation for the observed lack of satellites within 0-30degrees of their host galaxy's disc. The computations furthermore indicate that the evolution of the orbital eccentricity e is dependent on its initial value e (t =0) and the DMH's shape. We also discuss some implications of flattened DMHs for satellite debris streams.