Kavli Affiliate: Kohei Inayoshi
| First 5 Authors: Kohei Inayoshi, Roberto Maiolino, , ,
| Summary:
The James Webb Space Telescope (JWST) has uncovered low-luminosity active
galactic nuclei (AGNs) at high redshifts of $zgtrsim 4-7$, powered by
accreting black holes (BHs) with masses of $sim 10^{6-8}~M_odot$. One
remarkable distinction of these JWST-identified AGNs, compared to their
low-redshift counterparts, is that at least $sim 20%$ of them present
H$alpha$ and/or H$beta$ absorption, which must be associated with extremely
dense ($gtrsim 10^9~{rm cm}^{-3}$) gas in the broad-line region or its
immediate surroundings. These Balmer absorption features unavoidably imply the
presence of a Balmer break caused by the same dense gas. In this Letter, we
quantitatively demonstrate that a Balmer break can form in AGN spectra without
stellar components, when the accretion disk is heavily embedded in dense
neutral gas clumps with densities of $sim 10^{9-11}~{rm cm}^{-3}$, where
hydrogen atoms are collisionally excited to the $n=2$ states and effectively
absorb the AGN continuum at the bluer side of the Balmer limit. The non-stellar
origin of a Balmer break offers a potential solution to the large stellar
masses and densities inferred for little red dots (LRDs) when assuming that
their continuum is primarily due to stellar light. Our calculations indicate
that the observed Balmer absorption blueshifted by a few hundreds ${rm
km~s}^{-1}$ suggests the presence of dense outflows in the nucleus at rates
exceeding the Eddington value. Other spectral features such as higher
equivalent widths of broad H$alpha$ emission and presence of OI lines observed
in high-redshift AGNs including LRDs align with the predicted signatures of a
dense super-Eddington accretion disk.
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