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,…