The Bardeen-Petterson effect, disk breaking, and the spin orientations of supermassive black-hole binaries

Understand

Supermassive black-hole binaries are driven to merger by dynamical friction, loss-cone scattering of individual stars, disk migration, and gravitational-wave emission.

• Two main formation scenarios are expected.
• Binaries that form in gas-poor galactic environments do not experience disk migration and likely enter the gravitational-wave dominated phase with roughly isotropic spin orientations.
• Comparatively, binaries that evolve in gas-rich galactic environments might experience prominent phases of disk accretion, where the Bardeen-Petterson effect acts to align the spins of the black holes with the orbital angular momentum of the disk.