Kavli Affiliate: Roberto Maiolino
| 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 $sim 10^{6-8}~M_odot$. These AGN
populations are considered crucial for understanding early BH assembly and
coevolution with their host galaxies. 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 ($gtrsim 10^9$ cm$^{-3}$) gas along
the line of sight. 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 feature can form in AGN spectra
without stellar components, when the accretion disk is heavily embedded in
dense neutral gas clumps with densities of $sim 10^{9-11}$ 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 of
hydrogen-level populations indicate that the observed Balmer absorption
blueshifted by a few hundreds km s$^{-1}$ suggests the presence of dense
outflows at parsec scales in the nucleus. The outflow rate likely exceeds the
Eddington accretion rate, driven by powerful radiation from a super-Eddington
accretion disk. 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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