Dual black holes in merger remnants. II: spin evolution and gravitational recoil

TL;DR

This study uses high-resolution hydrodynamical simulations to investigate how the spins of merging black holes evolve within a circumnuclear disc, showing that accretion aligns spins with orbital angular momentum, resulting in low recoil velocities after merger.

Contribution

It demonstrates that accretion torques rapidly align black hole spins with orbital angular momentum, reducing gravitational recoil velocities in merger remnants.

Findings

Black hole spins align with orbital angular momentum within 1-2 Myr.

Residual spin misalignment remains below 10-30 degrees depending on disc temperature.

Recoil velocities are typically less than 70 km/s, much lower than isotropic spin distribution predictions.

Abstract

Using high resolution hydrodynamical simulations, we explore the spin evolution of massive dual black holes orbiting inside a circumnuclear disc, relic of a gas-rich galaxy merger. The black holes spiral inwards from initially eccentric co or counter-rotating coplanar orbits relative to the disc's rotation, and accrete gas that is carrying a net angular momentum. As the black hole mass grows, its spin changes in strength and direction due to its gravito-magnetic coupling with the small-scale accretion disc. We find that the black hole spins loose memory of their initial orientation, as accretion torques suffice to align the spins with the angular momentum of their orbit on a short timescale (<1-2 Myr). A residual off-set in the spin direction relative to the orbital angular momentum remains, at the level of <10 degrees for the case of a cold disc, and <30 degrees for a warmer disc. Alignment in a cooler disc is more effective due to the higher coherence of the accretion flow near each black hole that reflects the large-scale coherence of the disc's rotation. If the massive black holes coalesce preserving the spin directions set after formation of a Keplerian binary, the relic black hole resulting from their coalescence receives a relatively small gravitational recoil. The distribution of recoil velocities inferred from a simulated sample of massive black hole binaries has median <70 km/s much smaller than the median resulting from an isotropic distribution of spins.