The Stellar Orbital Structure in Axisymmetric Galaxy Models with Supermassive Black Hole Binaries

TL;DR

This study investigates the orbital dynamics in axisymmetric galaxy models with supermassive black hole binaries, revealing which stellar orbits interact with the binary and how they compare to those predicted in single SMBH models, highlighting the role of chaotic and resonant orbits.

Contribution

It provides a detailed comparison of stellar orbits in galaxies with single and binary SMBHs, clarifying which orbits actually interact with the binary and how they evolve.

Findings

Close match between predicted and actual interacting orbits in energy and angular momentum.

Binary SMBH influences a larger orbital reservoir due to its bigger radius of influence.

Nearly half of the interacting orbits are chaotic, with significant fractions being resonant.

Abstract

It has been well-established that particular centrophilic orbital families in non-spherical galaxies can, in principle, drive a black hole binary to shrink its orbit through three-body scattering until the black holes are close enough to strongly emit gravitational waves. Most of these studies rely on orbital analysis of a static SMBH-embedded galaxy potential to support this view; it is not clear, however, how these orbits transform as the second SMBH enters the center, so our understanding of which orbits actually interact with a SMBH binary is not ironclad. Here, we analyze two flattened galaxy models, one with a single SMBH and one with a binary, to determine which orbits actually do interact with the SMBH binary and how they compare with the set predicted in single SMBH-embedded models. We find close correspondence between the centrophilic orbits predicted to interact with the binary and those that are actually scattered by the binary, in terms of energy and Lz distribution, where Lz is the z component of a stellar particle's angular momentum. Of minor note: because of the larger mass, the binary SMBH has a radius of influence about 4 times larger than in the single SMBH model, which allows the binary to draw from a larger reservoir of orbits to scatter. Of the prediction particles and scattered particles, nearly half have chaotic orbits, 40% have fx:fy=1:1 orbits, 10% have other resonant orbits.