Dynamical Friction: A Global Picture

Dynamical friction is an astrophysical process responsible for a broad array of important phenomena: the inspiral of satellite galaxies into larger ones; mass segregation of heavy stars/remnants in dense star clusters; the early stages of binary supermassive black hole mergers.  Dynamical friction is generally modeled and conceptualized with the local formalism of Chandrasekhar, which provides a simple analytic expression for modeling the retarding force produced when a massive perturber moves through an infinite and uniform sea of small particles (e.g. stars, gas, dark matter).  These particles are focused into an overdense wake behind the perturber, and the self-gravity of the wake decelerates the massive object.  While the Chandrasekhar picture works remarkably well over a wide range of physical scales, its idealizations break down severely in certain circumstances.  The most notable such case concerns the inspiral of massive objects into nearly constant-density cores; both N-body simulations and observations (of e.g. the Fornax dwarf galaxy globular clusters) show a dramatic, perhaps total, reduction of the dynamical friction force below the Chandrasekhar predictions in this context.  In this talk, I will explain the "core stalling" phenomenon using a more rigorous global formulation of dynamical friction due to Tremaine and Weinberg.  By linearly perturbing the collisionless Boltzmann equation, one can compute the global response of a gravitating medium (e.g. a galaxy) to the influence of a massive perturber, enabling the study of density wakes in a non-uniform, non-infinite system.  In the global picture, the entire dynamical friction torque is produced by a small subset of stars in orbital resonance with the perturber.  Core stalling arises due to the depletion of all low-order resonances in the special harmonic potential of constant-density cores.  Interestingly, a previously neglected family of high-order resonances survives, producing a residual dynamical friction force at about 1/10th of the Chandrasekhar value.