Too many or too massive? Investigating the high- z demography of active SMBHs from JWST

Abstract Recent JWST observations have unveiled a numerous population of low-luminosity active galactic nuclei (AGN) at 4 ≲ z ≲ 10, with space densities roughly an order of magnitude above pre-JWST estimates, and many of these AGN have masses orders of magnitude above the local black hole mass–stellar mass (MBH − M⋆) scaling relations. We investigate the consistency of these observations within a data-driven framework that links the galaxy stellar mass function to the supermassive black hole (SMBH) mass function and AGN luminosity functions using different MBH − M⋆ relations and the observed Eddington-ratio distribution. By comparing our predictions against observed AGN luminosity functions at z ∼ 5.5 we find that observations can be reproduced either by highly-elevated MBH − M⋆ relations paired with low duty cycles (fAGN ∼ 0.08), or moderate relations with higher duty cycles (fAGN ∼ 0.5). Through the Sołtan argument, we find that MBH − M⋆ relations that are modestly above the local relation for AGN produce consistency between multiple tracers of the SMBH demography at z ∼ 5.5, while more extreme normalisations would require a weakly-evolving luminosity function at z ≥ 5.5. Continuity-equation modelling shows that initially high MBH − M⋆ relations predict a strong two-phase evolutionary scenario and very steep low-mass SMBH mass functions in tension with several current estimates, while more moderate relations generate local SMBH mass functions in better agreement with present determinations and near-constant scaling relations. Our results favour a scenario where SMBHs at z ∼ 5 on average lie modestly above local AGN scaling relations, with elevated but physically plausible duty cycles. Future wide-field clustering and demographic studies will help break the remaining degeneracies between SMBH scaling relations and AGN duty cycles at early cosmic times.