A novel subgrid model for super-Eddington accretion of spinning black holes in galaxy-scale simulations

ABSTRACT Super-Eddington accretion has been proposed to explain the existence of black holes (BHs) with masses exceeding a billion solar masses within the first billion years after the big bang. We present a novel accretion disc-based subgrid model for BH mass and spin evolution in the super-Eddington regime, implemented in the hydrodynamics code gizmo. In our model, motivated by results of radiation-hydrodynamics simulations of accretion discs, the growth of the BH is mediated by a subgrid accretion disc, comprising an inner photon-trapping region described by simulation-based fitting formulae and an outer thin $\alpha$-disc with three regions. We incorporate a self-consistent spin evolution prescription that transitions between the Bardeen–Petterson effect and inner thick-disc precession, depending on the accretion rate. We perform a suite of idealized simulations of a BH embedded in a gaseous circumnuclear disc and a spherically distributed stellar component to explore the conditions under which super-Eddington accretion can be sustained in the environment of a realistic galactic nucleus. Simulations with misaligned gas inflows onto an initially aligned BH–disc system yield very high Eddington ratios, triggered by the rapid removal of disc angular momentum via inflows. These results highlight the importance of angular momentum misalignment in enabling super-Eddington accretion and suggest that such episodes are difficult to trigger unless the system resides in a highly dynamical environment – a condition more likely to occur in high-redshift galaxies. Our model potentially provides a way to grow moderate-mass BH seeds to the sizes required to explain the bright high-redshift quasars.