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

ABSTRACT 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 expected. Binaries that form in gas-poor galactic environments do not experience migration likely enter the wave-dominated phase with roughly isotropic spin orientations. Comparatively, evolve gas-rich might prominent phases accretion, where Bardeen–Petterson effect acts align spins holes orbital angular momentum disc. However, if accretion breaks, alignment is expected be strongly suppressed – a phenomenon was recently shown occur large portion parameter space. In this paper, we develop semi-analytical model joint gas-driven supermassive taking into account impact breaking for first time. Our predicts occurrence distinct subpopulations depending on efficiency alignment. This implies future observations merging could potentially used (i) discriminate between hosts (ii) constrain dynamics warped discs.