The implications of overmassive black holes at $z > 5$ for quasar and black hole growth

Abstract

Recent JWST surveys of high-redshift galaxies have found surprisingly large black holes, with many being measured to be $\sim100$ times more massive than local galaxies with the same stellar mass. Here, we find that a population of these black holes would have dramatic implications for our understanding of their growth across cosmic time. We first show that the global black hole mass density at $z \sim 5$ would be comparable to local values. This would not occur if these black holes occupy a small fraction of galaxies, though it would be expected if these black holes radiate at high efficiencies (requiring that the central engines of AGN spin rapidly). We then show that the individual detected $z \sim 5$ black holes would remain overmassive compared to the local relation if they grow according to the average rates of state-of-the-art models. These systems must instead grow at least an order of magnitude more slowly than expected if they are to fall within the observed scatter of the local black hole mass-stellar mass relation. Such slow growth is surprising in comparison to other estimates of the radiative efficiency of AGN, especially because growth must be rapid at $z > 5$ in order to build up such massive black holes quickly. Finally, we highlight the challenges that overmassive black holes have on our understanding of the impact of quasar feedback on galaxies.