Black holes in the shadows: The missing high-ionization lines in the earliest JWST active galactic nuclei

¹ Dipartimento di Fisica “G. Occhialini”, Università degli Studi di Milano-Bicocca Piazza della Scienza 3 I-20126 Milano, Italy
² Stockholm University, Department of Astronomy and Oskar Klein Centre for Cosmoparticle Physics AlbaNova University Centre SE-10691 Stockholm, Sweden
³ Kavli Institute for Cosmology, University of Cambridge Madingley Road Cambridge CB3 0HA, UK
⁴ Department of Astronomy & Astrophysics, University of California Santa Cruz 1156 High Street Santa Cruz CA 95064, USA
⁵ Cavendish Laboratory, University of Cambridge 19 JJ Thomson Avenue Cambridge CB3 0HE, UK
⁶ Department of Physics and Astronomy, University College London Gower Street London WC1E 6BT, UK
⁷ Waseda Research Institute for Science and Engineering, Faculty of Science and Engineering, Waseda University, 3-4-1 Okubo Shinjuku Tokyo 169-8555, Japan

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 14 October 2025
Accepted: 15 December 2025

Abstract

Observations with the James Webb Space Telescope (JWST) have uncovered a substantial population of high-redshift, broad-line active galactic nuclei (AGNs), whose properties challenge standard models of black-hole growth and AGN emission. We analyzed a spectroscopic sample of 34 Type 1 AGNs from the JWST Advanced Deep Survey (JADES) survey, spanning redshifts 1.7 < z < 9, to constrain the physical nature of the accretion flows powering these sources with broad-line diagnostics statistically for the first time. At z > 5, we find a marked suppression of high-ionization emission lines (HeII, CIV, NV) relative to prominent broad Hα and narrow [OIII] features. This contrast places strong constraints on the shape of the ionizing spectral energy distribution (SED) and on the physical conditions in the broad-line region (BLR). By comparing the observations to photoionization models based on SEDs of black holes accreting at sub-Eddington ratios, we show that standard AGN continua struggle to reproduce the observed broad-line ratios and equivalent widths (EWs) across a wide ionization parameter range. These results suggest the need for modified SEDs – either intrinsically softened due to super-Eddington accretion or radiative inefficiencies in the innermost disk, or externally filtered by intervening optically thick gas that absorbs or scatters the highest energy photons before they reach the BLR.