Orbits Around Black Holes in Triaxial Nuclei

TL;DR

This paper explores the properties of pyramid orbits around supermassive black holes in triaxial nuclei, highlighting their role in feeding black holes and the impact of relativistic effects on orbit eccentricity.

Contribution

It introduces the analysis of pyramid orbits in nonaxisymmetric nuclei, deriving their equations of motion and examining relativistic limits on orbit eccentricity.

Findings

Pyramid orbits can approach arbitrarily close to the black hole.

Relativistic precession limits the maximum eccentricity of orbits.

Star capture on pyramid orbits can significantly feed black holes over cosmic timescales.

Abstract

We discuss the properties of orbits within the influence sphere of a supermassive black hole (BH), in the case that the surrounding star cluster is nonaxisymmetric. There are four major orbit families; one of these, the pyramid orbits, have the interesting property that they can approach arbitrarily closely to the BH. We derive the orbit-averaged equations of motion and show that in the limit of weak triaxiality, the pyramid orbits are integrable: the motion consists of a two-dimensional libration of the major axis of the orbit about the short axis of the triaxial figure, with eccentricity varying as a function of the two orientation angles, and reaching unity at the corners. Because pyramid orbits occupy the lowest angular momentum regions of phase space, they compete with collisional loss cone repopulation and with resonant relaxation in supplying matter to BHs. General relativistic advance of the periapse dominates the precession for sufficiently eccentric orbits, and we show that relativity imposes an upper limit to the eccentricity: roughly the value at which the relativistic precession time is equal to the time for torques to change the angular momentum. We argue that this upper limit to the eccentricity should apply also to evolution driven by resonant relaxation, with potentially important consequences for the rate of extreme-mass-ratio inspirals in low-luminosity galaxies. In giant galaxies, we show that capture of stars on pyramid orbits can dominate the feeding of BHs, at least until such a time as the pyramid orbits are depleted; however this time can be of order a Hubble time.