Loss Cone Dynamics

TL;DR

This paper reviews the complex dynamical processes that lead stars and remnants to be captured or disrupted by supermassive black holes, highlighting the influence of galaxy shape, relaxation times, and relativistic effects on capture rates.

Contribution

It provides a comprehensive review of loss cone dynamics, emphasizing the impact of galaxy geometry, relaxation processes, and relativistic precession on capture rates and the challenges in modeling these phenomena.

Findings

Higher capture rates in non-spherical galaxies due to torquing effects.

Relativistic precession affects the dynamics of compact object capture.

Capture rate estimates are uncertain in non-collisional, long relaxation time nuclei.

Abstract

Supermassive black holes can capture or disrupt stars that come sufficiently close. This article reviews the dynamical processes by which stars or stellar remnants are placed onto loss-cone orbits and the implications for feeding rates. The capture rate is well defined for spherical galaxies with nuclear relaxation times that are shorter than the galaxy's age. However, even the dense nucleus of the Milky Way may be less than one relaxation time old, and this is certainly the case for more massive galaxies; the capture rate in such galaxies is an initial-value problem with poorly-known initial conditions and the rate can be much higher, or much lower, than the rate in a collisionally relaxed nucleus. In nonspherical (axisymmetric, triaxial) galaxies, torquing of orbits by the mean field can dominate perturbations due to random encounters, leading to much higher capture rates than in the spherical geometry, particularly in (massive) galaxies with long central relaxation times. Relativistic precession plays a crucial role in mediating the capture of compact remnants from regions very near to the black hole, by destroying the orbital correlations that would otherwise dominate the torques. The complex dynamics of relativistic loss cones are not yet well enough understood for accurate estimates of compact-object (EMRI) capture rates to be made.