Massive black hole binaries in gaseous nuclear discs

TL;DR

This study uses high-resolution simulations to investigate how massive black hole pairs evolve in gaseous nuclear discs, revealing core formation, orbital circularization, and accretion behaviors during binary formation.

Contribution

It provides new insights into the dynamical evolution and accretion processes of black hole binaries in gaseous nuclear environments, highlighting the impact of disc perturbations and rotation.

Findings

Black hole binaries stall at ~5 pc due to disc core formation.

Orbital circularization occurs efficiently in co-rotating discs.

Gas accretion is highly dependent on black hole orbital properties.

Abstract

We study the evolution of a massive black hole pair in a rotationally supported nuclear disc. The distributions of stars and gas mimic the nuclear region of a gas-rich galaxy merger remnant. Using high-resolution SPH simulations, we follow the black hole dynamics and trace the evolution of the underlying background, until the black holes form a binary. We find that the gravitational perturbation of the pair creates a core in the disc density profile, hence decreasing the gas-dynamical drag. This leads the newly formed binary to stall at a separation of ~5 pc. In the early phases of the sinking, black holes lose memory of their initial orbital eccentricity if they co-rotate with the disc, as rotation of the gaseous background promotes circularization of the black hole orbits. Circularization is efficient until the black holes bind in a binary, though in the latest stages of the simulations a residual eccentricity > 0.1 is still present. Black holes are treated as sink particles, allowing for gas accretion. We find that accretion strongly depends on the dynamical properties of the black holes, and occurs preferentially after circularization.