The OBSIDIAN model: three regimes of black hole feedback

In theoretical models of galaxy evolution, black hole feedback is a necessary ingredient in order to explain the observed exponential decline in number density of massive galaxies. Most contemporary black hole feedback models in cosmological simulations rely on a constant radiative efficiency (usually η ∼ 0.1) at all black hole accretion rates. We present the OBSIDIAN subgrid model, a synthesis model for the spin-dependent radiative efficiencies of three physical accretion rate regimes, i.e. η = η(j, Ṁ_acc), for use in large-volume cosmological simulations. The three regimes include: an advection-dominated accretion flow (Ṁ_acc < 0.03 Ṁ_Edd), a quasar-like mode (0.03 < Ṁ_acc/Ṁ_Edd < 0.3), and a slim disc mode (Ṁ_acc > 0.3 Ṁ_Edd). Additionally, we include a large-scale powerful jet at low accretion rates. The black hole feedback model we present is a kinetic model that prescribes mass loadings but could be used in thermal models directly using the radiative efficiency. We implement the OBSIDIAN model into the SIMBA galaxy evolution model to determine if it is possible to reproduce galaxy populations successfully, and provide a first calibration for further study. Using a 2 × 1024³ particle cosmological simulation in a (150 cMpc)³ volume, we found that the model is successful in reproducing the galaxy stellar mass function, black hole mass–stellar mass relationship, and stellar mass–halo mass relationship. Moving forward, this model opens new avenues for exploration of the impact of black hole feedback on galactic environments.