Extremely Dense Gas around Little Red Dots and High-redshift AGNs: A Non-stellar Origin of the Balmer Break and Absorption Features

Abstract

The James Webb Space Telescope (JWST) has uncovered low-luminosity active galactic nuclei (AGNs) at high redshifts of $z\gtrsim 4-7$, 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.