Black Hole Occupation Fraction: Dependence on Black Hole Mass Threshold, Environment, Resolution, and Redshift

Abstract We take advantage of the state-of-the-art semianalytic model FEGA25 , run on merger trees extracted from three dark-matter-only cosmological simulations, to study the relation between the black hole (BH) occupation fraction, f BH,occ , and galaxy stellar mass as a function of BH mass threshold, galaxy type, simulated volume, numerical resolution, sampled galaxy population, and redshift. FEGA25 includes an improved treatment of active galactic nucleus (AGN) feedback and does not impose a preexisting BH seed population: BHs grow naturally through quasar and radio modes. Starting from the prerequisite that FEGA25 reproduces the observed BH mass function from at least z = 2 to the present day, our analysis leads to several results. We find that f BH,occ increases with stellar mass but that its normalization and shape depend strongly on the adopted BH mass threshold and on the relative contribution of central and satellite galaxies. The relative behavior of central and satellite galaxies depends on the simulation box and BH mass threshold, while the global relation should be interpreted as a population-weighted quantity. We also find significant box-to-box variations, reflecting the combined impact of numerical resolution, simulated volume, and sampled galaxy population. The redshift evolution is not universal: YS50 and the NewCluster zoom-in simulation show a trend qualitatively similar to that reported by M. Tremmel et al., whereas larger-volume boxes show the opposite behavior. Finally, comparison with other studies shows that the inferred occupation fraction is highly sensitive to BH mass threshold, simulated volume, numerical resolution, and sampled galaxy population.