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

TL;DR

This paper presents a semi-analytic model of supermassive black-hole binary evolution, incorporating the effects of disk breaking on spin alignment, which can help interpret future gravitational-wave observations.

Contribution

It introduces the first semi-analytic model that accounts for disk breaking effects in the spin alignment of supermassive black-hole binaries.

Findings

Predicts distinct binary subpopulations based on spin alignment efficiency.

Suggests gravitational-wave data can distinguish between gas-rich and gas-poor environments.

Provides a framework to constrain accretion disk dynamics from observations.

Abstract

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. However, if the accretion disk 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-analytic model of joint gas-driven migration and spin alignment of supermassive black-hole binaries taking into account the impact of disk 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 disks.