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

Sophie Koudmani, Jan Scholtz, Anthony J. Taylor, Ignas Juodžbalis, Debora Sijacki, Rachel S. Somerville, Roberto Maiolino, Emma Curtis-Lake, Steven L. Finkelstein, Martin A. Bourne, Francesco D'Eugenio, Sophia Geris, Lucy R. Ivey, Hannah Übler

First listed 2026-07-30 | Last updated 2026-07-28

Abstract

The active black holes 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, the observable tail of a broader population, or suffer from systematic uncertainties in mass estimates. We introduce the AESOPICA project, a suite of twelve mid-volume ($L = 60\,\mathrm{Mpc}$) cosmological simulations based on the FABLE galaxy formation model, varying the black hole seed mass across the theoretical formation channels ($M_\mathrm{seed} = 10^{2}$-$10^{5} \, \mathrm{M_{\odot}}$), 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 $M_\mathrm{BH}$-$M_\mathrm{stellar}$ relation for inefficient accretion models. Notably, the selected AGN lie on the local, weakly evolving $M_\mathrm{BH}$-$σ_\mathrm{stellar}$ 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.

Short digest

AESOPICA uses twelve 60 Mpc FABLE-based cosmological simulations, spanning 10^2–10^5 solar-mass seeds, altered accretion and super-Eddington prescriptions, and supernova feedback, then passes the outputs through the BALMERSOPICA mock JWST broad-line selection pipeline. The bulk of JWST AGN can be produced from any seed mass if early accretion is sufficiently efficient, though light seeds require the most favorable boosted, bursty growth; broad-line selection itself preferentially recovers sources that look overmassive in the black hole–stellar mass plane. The selected objects nevertheless follow the local, weakly evolving black hole–stellar velocity-dispersion relation, favoring a black-hole-first picture in which the host stellar component is still assembling, while the low-mass black hole mass function and gas metallicity retain the clearest leverage on seed formation channels.

Key figures to inspect

• Figure 6. This is the core selection-effect figure: applying CEERS, JADES, and deeper mock broad-line limits produces an apparently overmassive black hole population, with heavy seeds selected as overmassive in all models and light seeds requiring SE-BoostMax growth.
• Figure 7. The selected AGN fall on the local black hole mass–stellar velocity-dispersion relation despite their offset in black hole mass–stellar mass, directly illustrating the paper's central black-hole-first interpretation.
• Figure 9. This figure identifies the low-mass black hole mass function as a practical seeding diagnostic, showing that heavy seeds can exceed efficient light-seed number densities by about an order of magnitude and that stacking or deeper broad-line surveys can recover this difference.
• Figure 11. Gas-phase metallicity versus black hole mass and black hole-to-stellar mass ratio provides the paper's second proposed route to distinguishing seeds, while also exposing tension between the simulated parameter space and several extremely metal-poor JWST AGN hosts.