The missing Fe II bump in faint JWST active galactic nuclei: Possible evidence of metal-poor broad-line regions at early cosmic times

¹ Dipartimento di Fisica e Astronomia, Università di Firenze, Via G. Sansone 1, 50019 Sesto Fiorentino, Firenze, Italy
² INAF – Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, I-50125 Firenze, Italy
³ Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge CB3 OHA, UK
⁴ Cavendish Laboratory – Astrophysics Group, University of Cambridge, 19 JJ Thomson Avenue, Cambridge CB3 OHE, UK
⁵ Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT UK,
⁶ Max-Planck-Institut für Extraterrestrische Physik (MPE), Gießenbachstraße 1, 85748 Garching, Germany
⁷ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, I-40129 Bologna, Italy

^⋆ Corresponding author: bartolomeo.trefoloni@unifi.it

Received: 29 October 2024
Accepted: 26 May 2025

Abstract

Recent JWST observations have revealed a large population of intermediate/low-luminosity active galactic nuclei (AGNs) at early times with peculiar properties, different from local AGNs or luminous quasars. To better understand the physical conditions in the broad-line regions (BLRs) of these early AGNs, we used the optical Fe II (4434–4684 Å) and the broad Hβemission, and the ratio between their equivalent widths R_Fe, as a probe on a purposefully assembled sample. Specifically, we gathered a sample of 31 high redshift (⟨z⟩ = 6.5) AGNs, observed by JWST, with broad Hβdetection both in the high and low luminosity regimes (respectively 14 faint AGNs and 17 quasars), to investigate their optical Fe II emission properties. In addition, we carefully selected control samples at lower z. We found that the population of faint AGNs (log(L_Hβ,br/(erg s⁻¹)) ≲ 44) exhibits a significantly lower Fe II emission than their local counterparts having a mean (median) R_Fe < 0.24 (0.12) versus R_Fe ≃ 0.90 (0.79) in the control sample. At the same time, the quasars at the epoch of reionisation observed by JWST present a Fe II emission profile that closely resembles that observed at z < 3. We argue that the weakness of the Fe II bump in the faint JWST AGNs might be due to the reduced metallicity of their broad line region (≲0.5 Z_⊙), while luminous quasars have already reached chemical maturity (∼Z_⊙ or higher). Lastly, we highlight an intriguing similarity between the spectral properties of the high redshift population of faint AGNs with those harboured in local metal poor dwarf galaxies.