Self-regulated black hole growth via momentum deposition in galaxy merger simulations

We perform hydrodynamical simulations of major galaxy mergers using new methods for calculating the growth of massive black holes (BH) in galactic nuclei and their impact on the surrounding galaxy. We model BH growth by including a subgrid model for accretion produced by angular momentum transport on unresolved scales. The impact of the BH's radiation on surrounding gas is approximated by depositing momentum into the ambient gas, which produces an outward force away from the BH. We argue that these phenomenological models for BH growth and feedback better approximate the interaction between the BH and dense gas in galaxies than previous models. We show that this physics leads to self-regulated BH growth: during the peak of activity, the accretion rate on to the BH is largely determined by the physics of BH feedback, not the subgrid accretion model. The BH significantly modifies the gas dynamics in the galactic nucleus (less than or similar to 300 pc), but does not generate large-scale galactic outflows. Integrated over an entire galaxy merger, BH feedback has little effect on the total number of stars formed, but is crucial for setting the BH's mass.