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Arxiv: The missing FeII bump in faint JWST AGN: possible evidence for metal-poor broad-line regions at early cosmic times Published: 10/29/2024 11:01:31 AM Updated: 10/29/2024 11:01:31 AM

Paper abstract: Recent JWST observations have revealed a large population ofintermediate/low-luminosity AGN at early times with peculiar properties,different from local AGN or luminous quasars. To better understand the physicalconditions 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 theirequivalent 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 inthe high and low luminosity regimes (respectively 14 faint AGN and 12 quasars),to investigate their optical FeII emission properties. In addition, wecarefully selected control samples at lower z. We found that the populationof faint AGN (\rm \log(L_{H \beta} / (erg \, s^{-1}))<~ 44) exhibits asignificantly lower FeII emission than their local counterparts (R_{Fe}<0.24versus R_{Fe}~eq0.85 in the control sample), while the quasars at theepoch of reionisation observed by JWST present a FeII emission profile thatclosely resembles that observed at z<3. We argue that the weakness of theFeII bump in the faint JWST AGN might be due to the reduced metallicity oftheir broad line region (<~ 0.5~Z_{\odot}), while luminous quasars havealready reached chemical maturity (~ Z{_\odot} or higher). Lastly, wehighlight an intriguing similarity between the spectral properties of the highredshift population of faint AGN with those harboured in local metal poor dwarfgalaxies.