Extremely Dense Gas around Little Red Dots and High-redshift Active Galactic Nuclei: A Nonstellar 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 ≳ 4–7, powered by accreting black holes with masses of ∼10 ^6−8 M _⊙ . One remarkable distinction of these JWST-identified AGNs, compared to their low-redshift counterparts, is that at least ∼20% of them present H α and/or H β absorption, which must be associated with extremely dense (≳10 ^9 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 ∼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 nonstellar 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 hundred km s ^−1 , which 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 α emission and presence of O i lines observed in high-redshift AGNs including LRDs align with the predicted signatures of a dense super-Eddington accretion disk.

Keywords

• Galaxy formation
• High-redshift galaxies
• Quasars
• Supermassive black holes