James Webb Space Telescope Feed Post

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

Paper 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 ~ 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 ~ 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 ~ 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.