The black hole population in low-mass galaxies in large-scale
  cosmological simulations
  Recent systematic searches for massive black holes (BHs) in local dwarf
galaxies led to the discovery of a population of faint Active Galactic Nuclei
(AGN). We investigate the agreement of the BH and AGN populations in the
Illustris, TNG, Horizon-AGN, EAGLE, and SIMBA simulations with current
observational constraints in low-mass galaxies. We find that some of these
simulations produce BHs that are too massive, and that the BH occupation
fraction at z=0 is not inherited from the simulation seeding modeling. In
simulations, the ability of BHs and their host galaxies to power an AGN depends
on the subgrid modeling of the BH and of the galaxy. The fraction of AGN in
low-mass galaxies is not used to calibrate the simulations, and thus is a true
prediction. AGN fractions at z=0 span two orders of magnitude at fixed galaxy
stellar mass in simulations, similarly to observational constraints, but
uncertainties and degeneracies affect both observations and simulations. The
agreement is difficult to interpret due to differences in BH masses between
simulations and observations, BH occupation fraction affected by numerical
choices, and an unknown obscured fraction of AGN. Our work advocates for more
thorough comparisons with observations in the low-mass regime to improve the
modeling of cosmological simulations, and our understanding of BH and galaxy
physics in this regime. The mass of BHs, their ability to efficiently accrete
gas, and the AGN occupation fraction in low-mass galaxies have important
implications for the build-up of the entire BH and galaxy populations with
time.
