Episodic super-Eddington accretion as a clue to overmassive black holes in the early Universe

ABSTRACT Early James Webb Space Telescope (JWST) observations are providing growing evidence for a ubiquitous population of accreting supermassive black holes (BHs) at high redshift, many of which appear overmassive compared to the empirically derived local scaling relation between BH mass and host galaxy stellar mass. In this study, we leverage predictions from the semi-analytical Cosmic Archaeology Tool (CAT) to reconstruct the evolutionary pathways for this overmassive BH population, investigating how they assemble over cosmic time and interact with their host galaxies. We find that the large $\rm {\it M}_{BH}{\!-\!}{\it M}_{star}$ ratios can be explained if light and heavy BH seeds grow by short, repeated episodes of super-Eddington accretion, triggered by major galaxy mergers. On average, we find that BH-galaxy co-evolution starts in earnest only at $z \lt 8$, when $\simeq 30{{\ \rm per\ cent}}$ of the final galaxy stellar mass has formed outside the massive BH host. Our model suggests that super-Eddington bursts of accretion last between $0.5\,\text{and}\,3$ Myr, resulting in a duty cycle of $1{\!-\!}4{{\ \rm per\ cent}}$ for the target BH sample. The boost in luminosity of BHs undergoing super-Eddington accretion helps to explain the luminosity function of active galactic nuclei observed by JWST. At the same time, a large population of these overmassive BHs is predicted to be inactive, with Eddington ratios $\lambda _{\rm Edd} \lt 0.05$, in agreement with recent observations.