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Extremely Dense Gas around Little Red Dots and High-redshift Active Galactic Nuclei: A Non-stellar Origin of the Balmer Break and Absorption Features
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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$\alpha$ and/or H$\beta$ 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$\alpha$ 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.
Forward citations
Cited by 12 Pith papers
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What you see is what you get: empirically measured bolometric luminosities of Little Red Dots
Directly integrating the observed spectra of two Little Red Dots shows the bolometric luminosity is dominated by rest-frame optical light, lowering implied black hole masses to about 10^5 to 10^7 solar masses.
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Radiation GRMHD Models of Accretion onto Stellar-Mass Black Holes: I. Survey of Eddington Ratios
Full-transport radiation GRMHD simulations show super-Eddington black hole accretion is geometrically thick, drives strong outflows, and radiates with very low efficiency (below about 0.5% at 150 times Eddington).
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ATLAS. II. Extremely High Incidence of Balmer Line Absorption with Predominant Blueshifts in LRDs: Statistical Insights through Comparison with Type 1 AGNs
Balmer-line absorption occurs in ~35% (14/40) of JWST little-red-dot AGNs, roughly 850x the rate in SDSS type-1 AGNs, with mostly slow blueshifted absorber velocities.
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Outflows in super-Eddington quasars drive clumpy circumgalactic medium and extended H$\alpha$ nebulae at $z \gtrsim 6$
In z≈6 quasar simulations, super-Eddington black-hole growth clears escape channels, ejects cold clumps that raise neutral-hydrogen covering, and powers extended Hα nebulae that shrink when the quasar is obscured.
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An (in)complete NIRSpec census of Balmer absorption in Type 1 AGN -- radiation-driven outflows in little red dots, quasars and variable stars
About 44% of little red dots have hydrogen-alpha absorption from outflowing gas, implying radiatively driven outflows rather than static atmospheres.
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Spatial decomposition of Little Red Dots with JWST/NIRSpec IFU into broad-line red cores and narrow-line blue host galaxies
Spatially resolved JWST/NIRSpec IFU spectroscopy of five z~5 Little Red Dots shows blue continuum and narrow lines from extended hosts and red continuum plus broad Balmer features from compact cores.
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Reduced Incidence of Little Red Dots at z < 3 from Number Density and Halo Mass Evolution
LRDs transition from underdense low-halo-mass environments at z>4 to typical galaxy conditions by z~3.5, with halo growth leading to larger sizes and SED changes that explain their disappearance at lower redshifts.
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Discovery of Multiply Ionized Iron Emission Powered by an Active Galactic Nucleus in a z~7 Little Red Dot
The z=6.68 Little Red Dot THRILS 46403 shows a 4.5 sigma [FeVII] line, the first robust coronal-line detection among z>5 LRDs, plus narrow Balmer absorption, supporting an AGN with direct sightlines to gas at or beyon...
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Do Little Red Dots Vary?
Super-Eddington accretion models can explain why little red dots show almost no variability, whereas standard sub-Eddington AGN variability models predict changes that should already have been seen.
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Investigating Little Red Dots with UV Excess: Are They the High-Redshift Siblings of Blue Hot DOGs?
Little Red Dots are not the high-redshift relatives of Blue-excess Hot DOGs; they have less dust obscuration, little hot dust, and likely a different power source.
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An upper limit of 10$^6$ M$_\odot$ in dust from ALMA observations in 60 Little Red Dots
Stacking 60 little red dots in ALMA 1.3 mm data yields a 3 sigma dust mass limit near 10^6 solar masses, ten times deeper than previous limits.
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Little Red reionization factories
LRDs may drive cosmic reionization: tidal fields are said to funnel intergalactic hydrogen into colliding streams at LRD sites, igniting starbursts that ionize the gas.
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