Abstract
ABSTRACT Massive black holes (BHs) inhabiting galactic nuclei can be described by two parameters only, i.e. mass and spin, that change through cosmic time in response to accretion and merger events. While most numerical simulations accurately track the BH mass, spin evolution is rarely taken into account. In this work, we implement and validate a self-consistent sub-grid model for the evolution of the BH mass and spin via gas accretion in the hydrodynamics code gizmo. The model assumes that accretion from resolved scales does not occur instantaneously but is mediated by a sub-grid geometrically thin α-disc. After validating our model semi-analytically, we test it in an idealized environment consisting of a circumnuclear disc, where gas accretion on to the accretion disc is consistently determined by gizmo. In the absence of any accretion-related feedback, the spin evolution closely traces that observed in the semi-analytical models, and depends on the free parameters of our implementation, such as the initial BH spin, angular momentum of the accretion disc, and radius at which the gas inflow circularizes. In gizmo, we also couple our model with the biconical-outflow model presented in a companion paper, wherein the feedback axis is always aligned with the BH spin. In this last case, the evolution of the central BH differs significantly from the previous cases, since the feedback process modifies the gas dynamics and its inflow rates from resolved scales. Such an interaction cannot be modelled by simple semi-analytical models and should be treated using full N-body hydrodynamical simulations.
Highlights
According to the no-hair conjecture (Israel 1967, 1968; Carter 1971; Hawking 1972; Robinson 1975), massive black holes (BHs) are thought to be completely characterized by three parameters: mass, charge, and spin
This results in a swifter BH spin evolution for smaller values of Rcirc/Rsg and the difference in the evolution is more evident the smaller Rcirc/Rsg is
Configurations with larger fEdd,0 correspond to lower initial Jdisc/JBH, resulting in more compact and denser discs with higher accretion rates, and leading to a faster evolution of the BH spin
Summary
According to the no-hair conjecture (Israel 1967, 1968; Carter 1971; Hawking 1972; Robinson 1975), massive black holes (BHs) are thought to be completely characterized by three parameters: mass, charge, and spin. The accretion process triggers a directional feedback aligned with the BH spin, the feedback could significantly alter the dynamics of the gas reservoir, modifying the following spin evolution in a non-linear fashion. This is a severe limitation of semi-analytical studies, that cannot follow in real time the impact of the spin evolution on the larger-scale gas dynamics. For this reason, we hereby present a new implementation for the coupled evolution of BH spins, unresolved accretion discs, and directional feedback in N -body, hydrodynamical simulations. The results of the tests and their discussion are presented in Sections 4 and 5, respectively
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