Classification of Stellar Orbits in Axisymmetric Galaxies

TL;DR

This study analyzes the orbital structure of an axisymmetric galaxy model to understand how galaxy shape influences SMBH binary coalescence, revealing a rich set of resonant orbits that facilitate black hole merging.

Contribution

It identifies and characterizes resonant orbits in an axisymmetric galaxy model that are absent in spherical models, highlighting their role in SMBH binary evolution.

Findings

Axisymmetric models contain seven times more potential loss cone orbits than spherical models.

Resonant orbits like saucers are identified, which can interact with SMBH binaries.

The mass of these orbits is about three times that of the SMBH, aiding in binary coalescence.

Abstract

It is known that two supermassive black holes (SMBHs) cannot merge in a spherical galaxy within a Hubble time; an emerging picture is that galaxy geometry, rotation, and large potential perturbations may usher the SMBH binary through the critical three-body scattering phase and ultimately drive the SMBH to coalesce. We explore the orbital content within an N-body model of a mildly- flattened, non-rotating, SMBH-embedded elliptical galaxy. When used as the foundation for a study on the SMBH binary coalescence, the black holes bypassed the binary stalling often seen within spherical galaxies and merged on Gyr timescales (Khan et al. 2013). Using both frequency-mapping and angular momentum criteria, we identify a wealth of resonant orbits in the axisymmetric model, including saucers, that are absent from an otherwise identical spherical system and that can potentially interact with the binary. We quantified the set of orbits that could be scattered by the SMBH binary, and found that the axisymmetric model contained nearly seven times the number of these potential loss cone orbits compared to our equivalent spherical model. In this flattened model, the mass of these orbits is roughly 3 times of that of the SMBH, which is consistent with what the SMBH binary needs to scatter to transition into the gravitational wave regime.