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

TL;DR

The paper investigates non-stellar origins for the Balmer break and Balmer-absorption features in JWST-detected high-redshift AGNs by modeling an AGN continuum embedded in extremely dense circumnuclear gas. Using CLOUDY line-transfer calculations for gas with $n_{ m H} \gtrsim 10^{9-11}$ cm$^{-3}$ and $A_V=3$ mag, it shows rapid collisional pumping into the $n=2$ level can produce a Balmer break blueward of 3646 Å, with the break strength modulated by covering fraction and nebular emission. It further links blueshifted Hα absorption to dense outflows, estimating outflow rates $\dot{M}_{\rm out} \sim 1.2\,M_\odot$ yr$^{-1}$ and a moderately super-Eddington feeding state $\dot{m}_{\rm in} \sim 7.8$, $\dot{m}_{\bullet} \sim 2.8$, and predicts radiative signatures such as Lyβ pumping and O I lines, consistent with JWST data. The non-stellar Balmer-break interpretation reduces the inferred stellar masses of LRDs to around $10^9\,M_\odot$, alleviating tensions with the $\Lambda$CDM stellar-mass density and offering a coherent picture of dense gas, rapid BH growth, and distinctive spectral features in the early universe.

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 $\sim 10^{6-8}~M_\odot$. One remarkable distinction of these JWST-identified AGNs, compared to their low-redshift counterparts, is that at least $\sim 20\%$ of them present H$α$ and/or H$β$ absorption, which must be associated with extremely dense ($\gtrsim 10^9~{\rm cm}^{-3}$) 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 $\sim 10^{9-11}~{\rm cm}^{-3}$, 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 indicate that the observed Balmer absorption blueshifted by a few hundreds ${\rm km~s}^{-1}$ 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 OI lines observed in high-redshift AGNs including LRDs align with the predicted signatures of a dense super-Eddington accretion disk.

Figures (5)

• Figure 1: Left: AGN SEDs attenuated through a gas slab with a visual extinction of $A_V=3$ mag with $Z=0.1~Z_\odot$. Each curve represents the case with different density ($10^7\leq n_{\rm H}/{\rm cm}^{-3} \leq 10^{11}$) and thickness. With high densities of $n_{\rm H}\simeq 10^{9-11}~{\rm cm}^{-3}$, the SEDs show a deep Balmer break at $\lambda_{\rm B,lim} = 3646~{\rm \AA}$. Two vertical lines indicate the wavelengths ($\lambda_{\rm B,blue}=3600~{\rm \AA}$ and $\lambda_{\rm B,red}=4000~{\rm \AA}$) used to quantify the Balmer-break strength. Right: Total AGN SEDs including the nebular emission with a covering fraction of $C=0.5$. For the cases with $n_{\rm H}=10^{9-11}~{\rm cm}^{-3}$, the nebular components are shown separately (dashed). The Balmer jump feature of the nebular spectrum weakens the Balmer break strength in the total SED when dense absorbers with $n_{\rm H}\gtrsim 10^{11}~{\rm cm}^{-3}$ surround the AGN with a high covering fraction ($C\gtrsim 0.5$).
• Figure 2: Profiles of the hydrogen level populations in the $n=2$ (solid) and $n=3$ (dashed) states as a function of slab thickness normalized by the total value $\Delta s$ for each density case; $n_{\rm H}=10^7-10^{11}~{\rm cm}^{-3}$. The values are normalized by the total density of hydrogen nuclei ($n_{\rm H}$), including both neutral and ionized states. As the density increases, the hydrogen is excited to the higher energy states. The ratio of $n=2$ states reaches $n_2/n_{\rm H}\simeq 10^{-6}$, which is the equilibrium value with $T\simeq 8000~{\rm K}$ through particle collisions.
• Figure 3: The Balmer break strength defined by $F_{\lambda}(\lambda_{\rm B,red})/F_{\lambda}(\lambda_{\rm B,blue})$ for $N_{\rm H}=5.4\times 10^{22}~{\rm cm}^{-2}$ (dashed) and $1.7\times 10^{23}~{\rm cm}^{-2}$ (solid). Two covering fractions are considered: $C=0.5$ (black) and $C=1$ (gray). For the fiducial case (black and dashed curve), the Balmer break strength reaches values of $\geq 2$ in the density range of $10^9 \lesssim n_{\rm H}/{\rm cm}^{-3} \lesssim 2\times 10^{10}$. With increasing column density, the Balmer break becomes more prominent due to the enhanced column density of atomic hydrogen in the $n=2$ state. These depths are consistent with those of six broad-line LRDs that show a Balmer break in the PRISM spectrum Furtak_2024Wang_2024bKokorev_2024bLabbe_2024b.
• Figure 4: The H$\alpha$ line profiles with FWHM$_{\rm broad}=3000~{\rm km~s}^{-1}$, FWHM$_{\rm narrow}=400~{\rm km~s}^{-1}$ and $r=0.3$. We explore two cases: $b=150~{\rm km~s}^{-1}$, $\Delta v=-200~{\rm km~s}^{-1}$, and $C=0.5$ (left panel) and $b=10~{\rm km~s}^{-1}$, $\Delta v=+50~{\rm km~s}^{-1}$, and $C=0.8$ (right panel). The solid and dashed curves show the total H$\alpha$ line profile and the broad component with absorption by dense gas clumps with a column density of $n=2$ atomic hydrogen, $N_{{\rm H},n=2}=10^{16}~{\rm cm}^{-2}$. The total line profile with a finite spectral resolution ($R\equiv \Delta \lambda/\lambda_0 = 1500$ and $500$) is overlaid, where the source redshift is set to $z=5$.
• Figure 5: Same as in Figure \ref{['fig:line_profile']}, but illustrating how the line shape changes with the width of the absorption feature, varying $b$ from $50~{\rm km~s}^{-1}$ to $400~{\rm km~s}^{-1}$. The other parameters are identical to those used in Figure \ref{['fig:line_profile']}.