Kavli Affiliate: Roberto Maiolino
| First 5 Authors: Bartolomeo Trefoloni, Xihan Ji, Roberto Maiolino, Francesco D’Eugenio, Hannah Übler
| Summary:
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 $rm Hbeta$ 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.
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