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

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 <ns0:sc>II</ns0:sc> (4434–4684 Å) and the broad Hβemission, and the ratio between their equivalent widths RFe, 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 <ns0:sc>II</ns0:sc> emission properties. In addition, we carefully selected control samples at lower z. We found that the population of faint AGNs (log(LHβ,br/(erg s−1)) ≲ 44) exhibits a significantly lower Fe <ns0:sc>II</ns0:sc> emission than their local counterparts having a mean (median) RFe < 0.24 (0.12) versus RFe ≃ 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 <ns0:sc>II</ns0:sc> emission profile that closely resembles that observed at z < 3. We argue that the weakness of the Fe <ns0:sc>II</ns0:sc> 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.