  In large-scale hydrodynamical cosmological simulations, the fate of massive
galaxies is mainly dictated by the modeling of feedback from active galactic
nuclei (AGN). The amount of energy released by AGN feedback is proportional to
the mass that has been accreted onto the BHs, but the exact sub-grid modeling
of AGN feedback differs in all simulations. Whilst modern simulations reliably
produce populations of quiescent massive galaxies at z<2, it is also crucial to
assess the similarities and differences of the responsible AGN populations.
Here, we compare the AGN population of the Illustris, TNG100, TNG300,
Horizon-AGN, EAGLE, and SIMBA simulations. The AGN luminosity function (LF)
varies significantly between simulations. Although in agreement with current
observational constraints at z=0, at higher redshift the agreement of the LFs
deteriorates with most simulations producing too many AGN of L_{x, 2-10
keV}~10^43-10^44 erg/s. AGN feedback in some simulations prevents the existence
of any bright AGN with L_{x, 2-10 keV}>=10^45 erg/s (although this is sensitive
to AGN variability), and leads to smaller fractions of AGN in massive galaxies
than in the observations at z<=2. We find that all the simulations fail at
producing a number density of AGN in good agreement with observational
constraints for both luminous (L_{x, 2-10 keV}~10^43-10^45 erg/s) and fainter
(L_{x, 2-10 keV}~10^42-10^43 erg/s) AGN, and at both low and high redshift.
These differences can aid us in improving future BH and galaxy subgrid modeling
in simulations. Upcoming X-ray missions (e.g., Athena, AXIS, and LynX) will
bring faint AGN to light and new powerful constraints. After accounting for AGN
obscuration, we find that the predicted number density of detectable AGN in
future surveys spans at least one order of magnitude across the simulations, at
any redshift.
