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

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.