Relaxation in stellar systems, and the shape and rotation of the inner dark halo

Abstract

Why do galactic bars rotate with high pattern speeds, when dynamical friction should rapidly couple the bar to the massive, slowly rotating dark halo? This long-standing paradox may be resolved by considering the dynamical interactions between the galactic disc and structures in the dark halo. Dynamical friction between small-scale halo structure and the disc spins up and flattens the inner halo, thereby quenching the dynamical friction exerted by the halo on the bar; at the same time the halo heats and thickens the inner disc, perhaps forming a rapidly rotating bulge. Two possible candidates for the required halo structures are massive black holes and tidal streamers from disrupted precursor haloes. More generally, gravitational scattering from phase-wrapped inhomogeneities represents a novel relaxation process in stellar systems, intermediate between violent relaxation and two-body relaxation, which can isotropize the distribution function at radii where two-body relaxation is not effective.