Kavli Affiliate: Debora Sijacki
| First 5 Authors: Xihan Ji, Roberto Maiolino, Hannah Übler, Jan Scholtz, Francesco D’Eugenio
| Summary:
Recent observations from JWST have revealed an abundant population of active
galactic nuclei (AGN) and so-called “Little Red Dots” (LRDs) at $2lesssim z
lesssim 11$, many of which are characterized by V-shaped UV-to-optical
continua with turnovers around the Balmer limit. The physical nature of these
LRDs is unclear, and it remains debated whether the peculiar spectral shape
originates from AGN, compact galaxies, or both. We present the analysis of new
NIRSpec-IFU data from the BlackTHUNDER JWST Large Programme and archival
NIRSpec-MSA data of a lensed LRD at $z=7.04$. The spectra confirm the presence
of a smooth Balmer break and a broad H$beta$ tracing the Broad Line Region
(BLR) of an AGN. The small velocity dispersion of the H$beta$ narrow component
indicates a small dynamical mass of the host galaxy of $M_{rm dyn}<4 times
10^8~M_{odot}$, which implies that the stellar population cannot contribute
more than 10% to the optical continuum. We show that the Balmer break can be
well described by an AGN continuum absorbed by very dense ($n_{rm H}sim
10^{10}~{rm cm^{-3}}$) and nearly dust-free gas along our line-of-sight
(possibly gas in the BLR or its surrounding). The same gas is expected to
produce H$beta$ absorption, at a level consistent with a tentative detection
($3sigma$) in the high-resolution spectrum. Such a non-stellar origin of the
Balmer break may apply to other LRDs, and would alleviate the issue of
extremely high stellar mass surface densities inferred in the case of a stellar
interpretation of the Balmer break. We note that this is a rare case of a black
hole that is overmassive relative to both the host galaxy stellar and dynamical
masses. We finally report indications of variability and the first attempt of
AGN reverberation mapping at such an early epoch.
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