Resonant Dynamical Friction Around a Super-Massive Black Hole: Analytical Description

Abstract

We derive an analytical model for the so-called phenomenon of `resonant dynamical friction', where a disc of stars around a super-massive black hole interacts with a massive perturber, so as to align its inclination with the disc's orientation. We show that it stems from a singular behaviour of the orbit-averaged equations of motion, which leads to a rapid alignment of the argument of the ascending node $Ω$ of each of the disc stars, with that of the perturber, $Ω_{\rm p}$, with a phase-difference of $90^\circ$. This phenomenon occurs for all stars whose maximum possible $\dotΩ$ (maximised over all values of $Ω$ for all the disc stars), is greater than $\dotΩ_{\rm p}$; this corresponds approximately to all stars whose semi-major axes are less than twice that of the perturber. The rate at which the perturber's inclination decreases with time is proportional to its mass and is shown to be much faster than Chandrasekhar's dynamical friction. We find that the total alignment time is inversely proportional to the root of the perturber's mass. This persists until the perturber enters the disc. The predictions of this model agree with a suite of numerical $N$-body simulations which we perform to explore this phenomenon, for a wide range of initial conditions, masses, \emph{etc.}, and are an instance of a general phenomenon. Similar effects could occur in the context of planetary systems, too.