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

Daniel Roberts, Francesco Shankar, Vieri Cammelli, Fabio Fontanot, Alessandro Trinca, Laura Bisigello, Elena Dalla Bonta, Hao Fu, Roberto Gilli, Andrea Grazian, Luca Graziani, Andrea Lapi, Nicola Menci, Jan Scholtz, Karthik Mahesh Varadarajan

First listed 2026-01-30 | Last updated 2026-01-30

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 ($M_{\rm BH}-M_{*}$) 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 $M_{\rm BH}-M_{*}$ 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 $M_{\rm BH}-M_{*}$ relations paired with low duty cycles, or moderate relations with higher duty cycles. Through the Soltan argument, we find that $M_{\rm BH}-M_{*}$ relations that are modestly above the local relation for AGN produce consistency between multiple tracers of the SMBH demography at $z\sim 5.5$, while more extreme normalisations would require a weakly-evolving luminosity function at $z> 5.5$. Continuity-equation modelling shows that initially high $M_{\rm BH}-M_{*}$ 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 $z \sim 5$ 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.

Short digest

Builds a data-driven bridge from the galaxy stellar mass function to the SMBH mass function and z≈5.5 AGN luminosity functions, varying the M_BH–M_* relation and the observed Eddington-ratio distribution. Finds the JWST faint-AGN counts can be matched either by very high M_BH–M_* with low duty cycles or by moderate M_BH–M_* with higher duty cycles. Using the Soltan argument plus continuity-equation modeling, the authors favor SMBHs at z∼5 sitting modestly above local AGN scaling relations with elevated but plausible duty cycles; extreme normalizations would demand weak LF evolution at z>5.5 and yield overly steep low-mass SMBH mass functions. This frames how to reconcile JWST AGN demographics with early SMBH growth and highlights where future wide-field clustering/demography can break the duty-cycle–scaling-relation degeneracy.

Key figures to inspect

• Figure 1: Inspect the bolometric LF comparison to see how their reference LF tracks the JWST faint end versus Shen (2020) and how the Schechter fit to Kokorev et al. (2024)—including the Greene et al. (2025) corrections—sets the normalization that drives inferred duty cycles.
• Figure 2: Follow the end-to-end pipeline linking the galaxy stellar mass function to active SMBH demographics; note precisely where M_BH–M_* and the λ_Edd prior enter, and how the Soltan constraint on mass density tightens the allowed parameter space.
• Figure 3: Compare RV15, P23, and L25 M_BH–M_* relations against the JWST faint-AGN samples to gauge the offset above the local relation and the scatter required; this visualizes the ‘too many vs too massive’ lever arm.
• Figure 4: Examine the BL-AGN “plane” versus the abundance-matching and bolometric-correction tracks; contrast filled (observed) and open (intrinsic) symbols to assess how extinction/bolometric choices shift sources and impact inferred λ_Edd and duty cycles.