Massive black hole binary evolution in gas-rich mergers

TL;DR

This paper investigates the dynamics of black hole binaries in gas-rich galaxy mergers, emphasizing the importance of gas dissipation in driving black holes to very small scales, with insights from high-resolution simulations.

Contribution

It provides new high-resolution simulation results showing how gas dissipation influences black hole inspiral and binary formation down to unprecedented small scales.

Findings

Gas dissipation accelerates black hole inspiral in major mergers.

Black hole binaries form at scales of about 5 parsecs in major mergers.

Binary black holes can inspiral down to 0.001 parsecs, with gas dissipation enabling decay to 0.1 parsecs.

Abstract

We report on key studies on the dynamics of black holes (BHs) in gas-rich galaxy mergers that underscore the vital role played by gas dissipation in promoting BH inspiral down to the smallest scales ever probed with use of high-resolution numerical simulations. In major mergers, the BHs sink rapidly under the action of gas-dynamical friction while orbiting inside the massive nuclear disc resulting from the merger. The BHs then bind and form a Keplerian binary on a scale of 5 pc. In minor mergers, BH pairing proceeds down to the minimum scale explored of 10-100 pc only when the gas fraction in the less massive galaxy is comparatively large to avoid its tidal and/or ram pressure disruption and the wandering of the light BH in the periphery of the main halo. Binary BHs enter the gravitational wave dominated inspiral only when their relative distance is typically of 0.001 pc. If the gas preserves the degree of dissipation expected in a star-burst environment, binary decay continues down to 0.1 pc, the smallest length-scale ever attained. Stalling versus hardening below 0.1 pc is still matter of deep investigations.