Alignment of supermassive black hole binary orbits and spins

TL;DR

This paper demonstrates that in gas-driven supermassive black hole binaries, the individual black hole spins tend to align with the binary orbital axis much faster than the orbital axis aligns with the circumbinary gas, leading to highly aligned spins and reduced merger recoil.

Contribution

It shows that black hole spins in supermassive binaries align with the orbital axis rapidly, influencing merger dynamics and recoil velocities, especially for near-equal mass ratios.

Findings

Black hole spins align with the binary orbital axis faster than the orbital axis aligns with incoming gas.

Secondary black hole spins align more rapidly than primary spins.

Aligned spins can significantly reduce recoil velocities after merger.

Abstract

Recent studies of accretion onto supermassive black hole binaries suggest that much, perhaps most, of the matter eventually accretes onto one hole or the other. If so, then for binaries whose inspiral from ~1 pc to 0.001 - 0.01 pc is driven by interaction with external gas, both the binary orbital axis and the individual black hole spins can be reoriented by angular momentum exchange with this gas. Here we show that, unless the binary mass ratio is far from unity, the spins of the individual holes align with the binary orbital axis in a time few-100 times shorter than the binary orbital axis aligns with the angular momentum direction of the incoming circumbinary gas; the spin of the secondary aligns more rapidly than that of the primary by a factor ~(m_1/m_2)^{1/2}>1. Thus the binary acts as a stabilizing agent, so that for gas-driven systems, the black hole spins are highly likely to be aligned (or counteraligned if retrograde accretion is common) with each other and with the binary orbital axis. This alignment can significantly reduce the recoil speed resulting from subsequent black hole merger.