Galactic nuclei evolution with spinning black holes: method and implementation

Supermassive black holes at the centre of galactic nuclei mostly grow in mass through gas accretion over cosmic time. This process also modifies the angular momentum (or spin) of black holes, both in magnitude and in orientation. Despite being often neglected in galaxy formation simulations, spin plays a crucial role in modulating accretion power, driving jet feedback, and determining recoil velocity of coalescing black hole binaries. We present a new accretion model for the moving-mesh code AREPO that incorporates (i) mass accretion through a thin α-disc and (ii) spin evolution through the Bardeen-Petterson effect. We use a diverse suite of idealized simulations to explore the physical connection between spin evolution and larger scale environment. We find that black holes with mass ≲10⁷ M_⊙ experience quick alignment with the accretion disc. This favours prolonged phases of spin-up, and the spin direction evolves according to the gas inflow on time-scales as short as ≲100 Myr, which might explain the observed jet direction distribution in Seyfert galaxies. Heavier black holes (≳10⁸ M_⊙) are instead more sensitive to the local gas kinematic. Here, we find a wider distribution in spin magnitudes: spin-ups are favoured if gas inflow maintains a preferential direction, and spin-downs occur for nearly isotropic infall, while the spin direction does not change much over short time-scales ∼100 Myr. We therefore conclude that supermassive black holes with masses ≳5 × 10⁸ M_⊙ may be the ideal testbed to determine the main mode of black hole fuelling over cosmic time.