Kavli Affiliate: Roberto Maiolino
| First 5 Authors: Ignas Juodžbalis, Xihan Ji, Roberto Maiolino, Francesco D’Eugenio, Jan Scholtz
| Summary:
JWST has discovered a large population of Active Galactic Nuclei (AGN) at
high redshift. Many of these newly discovered AGN have broad permitted lines
(typically H$alpha$), but are extremely weak in the X-rays. Here we present
the NIRSpec spectrum of the most extreme of these objects, GN-28074, an AGN at
$z=2.26$ with prominent Balmer, Paschen and HeI broad lines, and with the
highest limit on the bolometric to X-ray luminosity ratio among all
spectroscopically confirmed AGN in GOODS. This source is also characterized by
a mid-IR excess, most likely associated with the AGN torus’ hot dust. The high
bolometric luminosity and moderate redshift of this AGN allow us to explore its
properties more in depth relative to other JWST-discovered AGN. The NIRSpec
spectrum reveals prominent, slightly blueshifted absorption of H$alpha$,
H$beta$ and HeI$lambda$10830. The Balmer absorption lines require gas with
densities of $n_{rm H}> 10^8~{rm cm}^{-3}$, inconsistent with an ISM origin,
but fully consistent with clouds in the Broad Line Region (BLR). This finding
suggests that at least part of the X-ray weakness is due to high (Compton
thick) X-ray absorption by (dust-free) clouds in the BLR, or in its outer,
slowly outflowing regions. GN-28074 is also extremely radio-weak. The radio
weakness can also be explained in terms of absorption, as the inferred density
of the clouds responsible for H$alpha$ absorption makes them optically thick
to radio emission through free-free absorption. Alternatively, in this and
other JWST-discovered AGN, the nuclear magnetic field may have not developed
properly yet, resulting both in intrinsically weak radio emission and also lack
of hot corona, hence intrinsic X-ray weakness. Finally, we show that recently
proposed scenarios, invoking hyper-dense and ultra-metal-poor outflows or Raman
scattering to explain the broad H$alpha$, are completely ruled out.
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