Satellite decay in flattened dark matter haloes

Abstract

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, MaGalie (Boily, Kroupa and Peñarrubia 2001). We find that the range of survival times of satellites within a flattened DMH becomes of the order of 100% 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 satellites in an axisymmetric DMH may provide a natural explanation for the observed lack of satellites within (0-30) degrees of their host galaxy's disc (Holmberg 1969; Zaritsky and González 1999). The computations furthermore indicate that the evolution of the orbital eccentricity $e$ is highly dependent of its initial value e(t=0) and the DMH's shape. We also discuss some implications of flattened DMHs for satellite debris streams.