How to raise a supermassive black hole: interpreting early JWST AGN with the AESOPICA simulations

Abstract The active galactic nuclei (AGN) uncovered by JWST in the early Universe are highly abundant and seemingly overmassive with respect to local scaling relations, challenging standard models of black hole formation and growth. Yet it remains unclear whether they trace an efficient early growth channel, represent the observable tail of a broader population, or reflect systematic uncertainties in mass estimates. We introduce the aesopica project, a suite of twelve mid-volume (L = 60 cMpc) cosmological simulations based on the fable galaxy formation model, varying the black hole seed mass across the theoretical formation channels (Mseed = 102–105 M⊙), the accretion efficiency, including super-Eddington bursts, and the supernova feedback strength. We forward-model observational selection with balmersopica, a mock JWST broad-line survey pipeline that assesses the detectability of each simulated AGN for a given grating and exposure time. We find that the bulk of the JWST AGN population can be assembled from any seed mass provided the accretion efficiency is high, although light seeds require the most favourable accretion conditions explored. Applying broad-line selection naturally recovers the apparently overmassive population, with the detected AGN lying furthest above the intrinsic MBH–Mstellar relation for inefficient accretion models. The selected AGN lie on the local, weakly evolving MBH–σstellar relation, supporting a scenario where black holes assemble before the stellar component is fully established. Since efficient accretion rapidly erases the imprint of the initial seed mass, the low-mass end of the black hole mass function and host gas-phase metallicities offer the most promising discriminants between seeding channels.