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

Judah Luberto, Steven R. Furlanetto

First listed 2025-08-19 | Last updated 2025-08-19

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.

Short digest

Analyzing recent JWST reports of ~100× overmassive black holes at z>5, the authors show that if such BHs are common, the global black hole mass density at z≈5 would already rival today’s value—unless the overmassive systems are rare or accrete with very high radiative efficiencies (implying rapid spin). Tracking representative z≈5 detections forward using average growth rates from modern models keeps them overmassive relative to the local Mbh–M⋆ relation. To land within the local scatter by z≈0, these BHs must grow at least an order of magnitude more slowly than expected. This creates tension with the rapid early growth needed to assemble them and complicates standard expectations for quasar feedback on galaxies.

Key figures to inspect

• BH mass density vs. redshift under different assumptions (universal overmassive BHs, partial occupation fraction f, and varying radiative efficiency ε): verify that a universal overmassive population pushes z≈5 BHMD to near-local levels and identify which f–ε combinations avoid this.
• Evolutionary tracks on the Mbh–M⋆ plane for the observed z≈5 systems: check how fiducial Eddington ratios/duty cycles keep the tracks above the local relation and quantify the ≥10× growth slowdown required to reach the local scatter.
• Constraint map of radiative efficiency (or spin proxy) versus occupation fraction: read off the regions that reconcile high-z BHMD with z≈0 totals, highlighting the need for high ε (rapid spins) if the overmassive phase is widespread.
• Maximum allowed accretion histories (Eddington ratio or duty cycle vs. time/redshift): identify the upper limits on growth consistent with ending on the local Mbh–M⋆ relation by today.
• Integrated AGN feedback budget versus host properties: assess how overmassive early BHs alter expected feedback energy/momentum and the implied impact on galaxy growth.