Deformation of the Galactic Centre stellar cusp due to the gravity of a growing gas disc

TL;DR

This paper models how a growing gas disc around the Galactic Centre black hole deforms the surrounding stellar cusp, affecting star orbits and potentially leading to increased stellar envelope stripping.

Contribution

It introduces a linear theory framework to analyze the deformation of a spherical star cluster by a growing gas disc, providing new density profiles and insights into orbital dynamics.

Findings

The stellar cusp becomes spheroidally deformed with increased flattening near the black hole.

Orbits are altered to spend more time near the disc plane, affecting stellar interactions.

Linear theory explains circulating orbits; non-linear resonance capture needs further study.

Abstract

The nuclear star cluster surrounding the massive black hole at the Galactic Centre consists of young and old stars, with most of the stellar mass in an extended, cuspy distribution of old stars. The compact cluster of young stars was probably born in situ in a massive accretion disc around the black hole. We investigate the effect of the growing gravity of the disc on the orbits of the old stars, using an integrable model of the deformation of a spherical star cluster with anisotropic velocity dispersions. A formula for the perturbed phase space distribution function is derived using linear theory, and new density and surface density profiles are computed. The cusp undergoes a spheroidal deformation with the flattening increasing strongly at smaller distances from the black hole; the intrinsic axis ratio ~ 0.8 at ~ 0.15 pc. Stellar orbits are deformed such that they spend more time near the disc plane and sample the dense inner parts of the disc; this could result in enhanced stripping of the envelopes of red giant stars. Linear theory accounts only for orbits whose apsides circulate. The non-linear theory of adiabatic capture into resonance is needed to understand orbits whose apsides librate. The mechanism is a generic dynamical process, and it may be common in galactic nuclei.