Too many or too massive? Investigating the high- z demography of active SMBHs from JWST
Year: 2026
Type: article
Abstract: ABSTRACT Recent James Webb Space Telescope (JWST) observations have unveiled a numerous population of low-luminosity active galactic nuclei (AGNs) at $4\lesssim z\lesssim 10$, with space densities roughly an order of magnitude above pre-JWST estimates, and many of these AGNs have masses orders of magnitude above the local black hole mass–stellar mass ($M_{\rm BH}-M_{\star }$) 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 $M_{\rm BH}-M_{\star }$ relations and the observed Eddington-ratio distribution. By comparing our predictions against observed AGN luminosity functions at $z\sim 5.5$ we find that observations can be reproduced either by highly elevated $M_{\rm BH}-M_{\star }$ relations paired with low duty cycles ($f_{\rm AGN}\sim 0.08$), or moderate relations with higher duty cycles ($f_{\rm AGN}\sim 0.5$). Through the Sołtan argument, we find that $M_{\rm BH}-M_{\star }$ relations that are modestly above the local relation for AGNs produce consistency between multiple tracers of the SMBH demography at $z\sim 5.5$, while more extreme normalizations would require a weakly evolving luminosity function at $z\ge 5.5$. Continuity-equation modelling shows that initially high $M_{\rm BH}-M_{\star }$ 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 $z \sim 5$ 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.
Institutions: University of Southampton, University of Modena and Reggio Emilia, University of Trieste, Trieste Astronomical Observatory, Institute for Fundamental Physics of the Universe, Chalmers University of Technology, National Institute for Astrophysics, University of Insubria, Astronomical Observatory of Rome, Sapienza University of Rome, Osservatorio Astronomico di Padova, University of Padua, University of Lancashire, Fudan University, Osservatorio di Astrofisica e Scienza dello Spazio, Istituto Nazionale di Fisica Nucleare, Istituto Nazionale di Fisica Nucleare, Sezione di Roma I, Scuola Internazionale Superiore di Studi Avanzati, Istituto di Radioastronomia di Bologna, Istituto Nazionale di Fisica Nucleare, Sezione di Trieste, University of Cambridge
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: gold
Estimated APC paid: $3,046
Awards: G4895826563, G5789910615, G6207426046
OpenAlex ID: W7127424340