Black hole evolution – II. Spinning black holes in a supernova-driven turbulent interstellar medium

Supermassive black holes (BHs) accrete gas from their surroundings and coalesce with companions during galaxy mergers, and both processes change the BH mass and spin. By means of high-resolution hydrodynamical simulations of galaxies, either idealized or embedded within the cosmic web, we explore the effects of interstellar gas dynamics and external perturbations on BH spin evolution. All these physical quantities were evolved on the fly in a self-consistent manner. We use a ‘maximal’ model to describe the turbulence induced by stellar feedback to highlight its impact on the angular momentum of the gas accreted by the BH. Periods of intense star formation are followed by phases where stellar feedback drives large-scale outflows and hot bubbles. We find that BH accretion is synchronized with star formation, as only when gas is cold and dense do both processes take place. During such periods, gas motion is dominated by consistent rotation. On the other hand, when stellar feedback becomes substantial, turbulent motion randomizes gas angular momentum. However, BH accretion is strongly suppressed in that case, as cold and dense gas is lacking. In our cosmological simulation, at very early times (z > 6), the galactic disc has not yet settled and no preferred direction exists for the angular momentum of the accreted gas, so the BH spin remains low. As the gas settles into a disc (6 > z > 3), the BH spin then rapidly reaches its maximal value. At lower redshifts (z < 3), even when galaxy mergers flip the direction of the angular momentum of the accreted gas, causing it to counter-rotate, the BH spin magnitude only decreases modestly and temporarily. Should this be a typical evolution scenario for BH, it potentially has dramatic consequences regarding their origin and assembly, as accretion on maximally spinning BH embedded in thin Shakura–Sunyaev disc is significantly reduced.