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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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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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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 11 Pith papers
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Little red dots as a cosmological probe: constraining $H_0$ with quasi-periodic pulsations
Derives idealized P-L-T relation from hydrostatic envelope model for LRDs and uses sparse data to obtain preliminary H0 = 120.7 with large errors as proof-of-concept.
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(LRDs)$^2$: The Low-ReDshift Little Red Dots Survey. II. DESI DR1 Sample
The survey identifies 27 low-redshift LRDs with compact morphology, V-shaped continua, broad Balmer lines with extreme decrements, and ubiquitous outflows, matching high-z counterparts and yielding a number density lo...
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Hidden in Pixels I: Discovery of dual "little red dots" indicates excess clustering on kilo-parsec scales
Four dual LRD candidates at z~5.5 with kpc separations show 20-30x excess sub-arcsec clustering versus extrapolated AGN ACF, implying merger-driven SMBH growth.
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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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Reduced Incidence of Little Red Dots at z < 3 from Number Density and Halo Mass Evolution
Little red dots shift from underdense, low-halo-mass environments at z>4 to ordinary galaxy environments by z~3.5, explaining their declining abundance at z<3.
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A "Black Hole Star" Reveals the Remarkable Gas-Enshrouded Hearts of the Little Red Dots
A source 660 million years after the Big Bang is interpreted as a black hole star with a dust-free dense gas atmosphere, implying Little Red Dots have black hole masses overestimated by orders of magnitude.
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Gravitational Waves from the Cosmic Dawn: Tracing Cosmic Black Hole Binaries with ET, LGWA and LISA
Super-Eddington accretion boosts predicted LISA detections of high-redshift black hole binaries to ~64 per year while dropping ET detections to ~4 per year, compared to ~32 and ~64 under Eddington-limited growth.
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