On the Formation of S-stars from a Recent Massive Black Hole Merger in the Galactic Center

TL;DR

This paper demonstrates through N-body simulations that a recent black hole merger can produce the orbital characteristics of the S-star cluster near the Galactic Center, explaining their eccentric and inclined orbits.

Contribution

It introduces a dynamical formation scenario for S-stars involving a black hole merger and secular torques, a novel explanation for their observed properties.

Findings

Black hole merger causes a gravitational recoil that forms an eccentric disk.

Stars migrate inward and gain high eccentricities and inclinations.

The model suggests the black hole merger occurred within the last 10 million years.

Abstract

The Galactic Center hosts a rotating disk of young stars between 0.05 and 0.5 pc of Sgr A*. The ``S-stars'' at a distance $<0.04$ pc, however, are on eccentric orbits with nearly isotropically distributed inclinations. The dynamical origin of the S-star cluster has remained a theoretical challenge. Using a series of $N$-body simulations, we show that a recent massive black hole merger with Sgr A* can self-consistently produce many of the orbital properties of the Galactic nuclear star cluster within 0.5 pc. A black hole merger results in a gravitational wave recoil kick, which causes the surrounding cluster to form an apse-aligned, eccentric disk. We show that stars near the inner edge of an eccentric disk migrate inward and are driven to high eccentricities and inclinations due to secular torques similar to the eccentric Kozai-Lidov mechanism. In our fiducial model, starting with a thin eccentric disk with $e = 0.3$, the initially unoccupied region within $0.04$ pc is populated with high eccentricity, high inclination S-stars within a few Myr. This dynamical channel would suggest that a black hole of mass $2^{+3}_{-1.2} \times 10^5 \ M_{\odot}$ merged with Sgr A* within the last 10 Myr.