AGN Feedback Models and AGN Demographics I: Radio-Mode AGN in EAGLE, SIMBA and TNG100 are Inconsistent with Observations

Abstract

We compare predictions of how Active Galactic Nuclei (AGN) populate host galaxies at low redshifts to observations, finding large discrepancies between cosmological simulation predictions and observed patterns. Modern cosmological simulations include AGN feedback models tuned to reproduce the observed galaxy stellar mass function. However, due to a lack of real understanding of the physics of AGN feedback, these models vary significantly across simulations. To distinguish between the models and potentially test the underlying physics, we carry out independent tests of these models. In an earlier study, we found that $F_{\rm AGN}$ -- the observed completeness-corrected fraction of galaxies hosting radio AGN with an Eddington ratio $\lambda > 10^{-3}$ -- to be a strong function of host galaxy stellar mass ($M_\star$) but nearly independent of host specific star formation rates (sSFR) at fixed $M_\star$. In this study, we test the radio mode AGN feedback models of the EAGLE, SIMBA, and TNG100 simulations by comparing their predictions of $F_{\rm AGN} \left(M_\star \right)$ to our observational constraint. We find that none of these simulations even qualitatively reproduce the observed dependencies of $F_{\rm AGN}$ on $M_\star$ and sSFR. Finally, we find that although the given TNG100 model could be modified in order to better reproduce the observed $F_{\rm AGN}$ trend, this modification would likely also change its prediction for the local stellar mass function and star formation rates -- key observations used for calibrating the simulation in the first place. Our findings highlight a pressing need to revisit the AGN feedback prescriptions in EAGLE, SIMBA, TNG100 and other similar models.