Spin-dependent winds were associated with a marked change in the simulated evolution of a massive black-hole binary. In runs with feedback, the orbit first expanded and then stalled after about 10 binary orbits, while an initially misaligned spin remained substantially out of alignment. In runs without feedback, the orbit continued to expand and that spin approached the reference z > 0 axis within about 30 orbital periods.
What the model tracks
The calculation followed the binary's spin, mass and accretion alongside the gas in a circumbinary disc. It used GIZMO, a meshless hydrodynamics/N-body code, together with a sub-grid prescription for black-hole growth, spin evolution, accretion and feedback. The feedback was modeled as anisotropic kinetic winds: their angular distribution depended on black-hole spin orientation and magnitude.
The full suite comprised 10 simulations: six with feedback and four without. In the no-feedback runs, the semi-major axis—the size of the binary's orbit—increased along nearly identical tracks. The feedback runs also expanded initially, but all entered a clear stalling phase after approximately 10 orbits.
Accretion falls away
Accretion showed the same divide. In no-feedback runs, the Eddington factor, the simulation's normalized measure of accretion, remained approximately 0.05. With feedback, the Eddington ratio decreased by one to two orders of magnitude, a reported scale of change rather than a statistical uncertainty interval. It reached zero for both black holes after approximately 20 to 25 orbital periods in the reported feedback evolution.
The runs with feedback also showed changes in the discs around the individual holes. Those minidiscs—small discs around each black hole—were destroyed after 20 to 25 binary orbits. Before disruption, the minidisc progressively tilted toward the black hole's equatorial plane.
The spins stay out of line
The contrast extended to spin evolution. Without feedback, the initially misaligned spin approached the reference z > 0 axis within roughly 30 orbital periods. With feedback, alignment largely stalled and substantial misalignment remained through the end of the simulations.
The authors also ran an idealized isolation calculation using the measured feedback Eddington-factor history with coplanar discs. It closely reproduced the full feedback spin evolution, and the paper reports that this reduced-accretion calculation captured most of the delayed alignment. It was an idealized calculation rather than an independent simulation condition.
A longer, quieter cycle
One run was extended to about 70 binary orbits. It showed alternating active and quiescent phases, with most of the binary's time spent in low-density, quiescent conditions. The length of those phases may depend on how the model decides when feedback restarts and on how gas is fed into the system.
What remains uncertain
Because the evidence comes from a numerical suite—six feedback simulations and four without—it describes the behavior of the modeled binary and disc, not an observational sample of actual binaries.
The resolution check supported the spin result: the higher-resolution spin evolution closely followed the fiducial-resolution run. The reference orbital evolution, however, was not fully converged at the tested resolutions, leaving a numerical caveat around the detailed orbital track.
Paper data and sources
Original title: Winds Against Alignment: AGN Feedback and the Spin Evolution of Massive Black Hole Binaries
Authors: Francesco Bollati, Marta Volonteri, Alessandro Lupi et al.
Journal/Repository: arXiv
Status: Preprint, not yet peer-reviewed
First online: 2026-08-26
DOI: Not available
Original paper · Full text