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 ii (4434–4684 Å) and the broad Hβ emission, 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 Hβ detection 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(LHβ/(erg s−1)) ≲ 44) exhibits a significantly lower Fe ii 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 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.