The missing FeII bump in faint JWST AGN: possible evidence for metal-poor broad-line regions at early cosmic times

TL;DR

This study investigates the FeII emission in early universe AGN observed by JWST, revealing that faint AGN have weaker FeII emission likely due to lower metallicity, contrasting with more mature quasars.

Contribution

It provides the first detailed analysis of FeII emission in high-redshift faint AGN, suggesting metal-poor broad-line regions at early cosmic times.

Findings

Faint JWST AGN show significantly lower FeII emission than local counterparts.

Luminous quasars at high redshift have FeII emission similar to lower redshift quasars.

Weak FeII in faint AGN likely indicates reduced metallicity in their broad line regions.

Abstract

Recent JWST observations have revealed a large population of intermediate/low-luminosity AGN at early times with peculiar properties, different from local AGN or luminous quasars. To better understand the physical conditions in the BLRs of these early AGN, we used the optical FeII (4434--4684 \AA) and the broad $\rm H \beta$ emission, and the ratio between their equivalent widths $R_{Fe}$, as a probe on a purposefully assembled sample. Specifically, we gathered a sample of 26 high redshift ($\langle z \rangle$=6.4) AGN, observed by JWST, with broad $\rm H\beta$ detection both in the high and low luminosity regimes (respectively 14 faint AGN and 12 quasars), to investigate their optical FeII emission properties. In addition, we carefully selected control samples at lower $z$. We found that the population of faint AGN ($\rm \log(L_{H \beta} / (erg \, s^{-1}))\lesssim 44$) exhibits a significantly lower FeII emission than their local counterparts ($R_{Fe}<$0.24 versus $R_{Fe}\simeq$0.85 in the control sample), while the quasars at the epoch of reionisation observed by JWST present a FeII emission profile that closely resembles that observed at $z<3$. We argue that the weakness of the FeII bump in the faint JWST AGN might be due to the reduced metallicity of their broad line region ($\lesssim 0.5~Z_{\odot}$), while luminous quasars have already reached chemical maturity ($\sim Z{_\odot}$ or higher). Lastly, we highlight an intriguing similarity between the spectral properties of the high redshift population of faint AGN with those harboured in local metal poor dwarf galaxies.