Black hole mergers: do gas discs lead to spin alignment?

TL;DR

This paper investigates how gas discs influence the spin alignment of merging supermassive black holes, finding that nonlinear warp propagation can significantly hinder alignment, especially for highly spinning black holes, affecting recoil velocities.

Contribution

It introduces improved estimates of spin alignment considering nonlinear warp effects and system parameters, highlighting the reduced likelihood of alignment for high-spin black holes.

Findings

Approximately 40% of high-spin black holes may not align with the disc before merger.

Nonlinear warp propagation diminishes the disc's ability to communicate warps and align spins.

High-spin black holes are more prone to strong recoil velocities after merger.

Abstract

In this Letter we revisit arguments suggesting that the Bardeen-Petterson effect can coalign the spins of a central supermassive black hole binary accreting from a circumbinary (or circumnuclear) gas disc. We improve on previous estimates by adding the dependence on system parameters, and noting that the nonlinear nature of warp propagation in a thin viscous disc affects alignment. This reduces the disc's ability to communicate the warp, and can severely reduce the effectiveness of disc-assisted spin alignment. We test our predictions with a Monte Carlo realization of random misalignments and accretion rates and we find that the outcome depends strongly on the spin magnitude. We estimate a generous upper limit to the probability of alignment by making assumptions which favour it throughout. Even with these assumptions, about 40% of black holes with $a \gtrsim 0.5$ do not have time to align with the disc. If the residual misalignment is not small and it is maintained down to the final coalescence phase this can give a powerful recoil velocity to the merged hole. Highly spinning black holes are thus more likely of being subject to strong recoils, the occurrence of which is currently debated.