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The sensitivity of X-ray facilities and our ability to detect fainter activegalactic nuclei (AGNs) will increase with the upcoming Athena mission and theAXIS and Lynx concept missions, thus improving our understanding ofsupermassive black holes (BHs) in a luminosity regime that can be dominated byX-ray binaries. We analyze the population of faint AGN (L_x (2-10 keV) < 10^42erg/s) in the Illustris, TNG100, EAGLE, and SIMBA cosmological simulations, andfind that the properties of their host galaxies vary from one simulation toanother. In Illustris and EAGLE, faint AGN are powered by low-mass BHs locatedin low-mass star-forming galaxies. In TNG100 and SIMBA, they are mostlyassociated with more massive BHs in quenched massive galaxies. We model theX-ray binary populations (XRB) of the simulated galaxies, and find that AGNoften dominate the galaxy AGN+XRB hard X-ray luminosity at z>2, while XRBsdominate in some simulations at z<2. Whether the AGN or XRB emission dominatesin star-forming and quenched galaxies depends on the simulations. Thesedifferences in simulations can be used to discriminate between galaxy formationmodels with future high-resolution X-ray observations. We compare theluminosity of simulated faint AGN host galaxies to observations of stackedgalaxies from Chandra. Our comparison indicates that the simulationspost-processed with our X-ray modeling tend to overestimate the AGN+XRB X-rayluminosity; luminosity that can be strongly affected by AGN obscuration. Somesimulations reveal clear AGN trends as a function of stellar mass (e.g., galaxyluminosity drop in massive galaxies), which are not apparent in theobservations.

Bringing faint active galactic nuclei (AGNs) to light: a view from large-scale cosmological simulations