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

Supermassive black hole binaries are driven to merger by dynamical friction, loss-cone scattering of individual stars, disc migration, and gravitational wave emission. Two main formation scenarios are expected. Binaries that form in gas-poor galactic environments do not experience disc 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 disc accretion, where the Bardeen-Petterson effect acts to align the spins of the black holes with the orbital angular momentum of the disc. However, if the accretion disc breaks, alignment is expected to be strongly suppressed - a phenomenon that was recently shown to occur in a large portion of the parameter space. In this paper, we develop a semi-analytical model of joint gas-driven migration and spin alignment of supermassive black hole binaries taking into account the impact of disc breaking for the first time. Our model predicts the occurrence of distinct subpopulations of binaries depending on the efficiency of spin alignment. This implies that future gravitational wave observations of merging black holes could potentially be used to (i) discriminate between gas-rich and gas-poor hosts and (ii) constrain the dynamics of warped accretion discs.