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Little Red Dots at an Inflection Point: Ubiquitous "V-Shaped" Turnover Consistently Occurs at the Balmer Limit
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abstract
Among the most puzzling early discoveries of JWST are "Little Red Dots" -- compact red sources that host broad Balmer emission lines and, in many cases, exhibit a "V shaped" change in slope in the rest-optical. The physical properties of Little Red Dots currently have order-of-magnitude uncertainties, because models to explain the continuum of these sources differ immensely. Here, we leverage the complete selection of red sources in the RUBIES program, supplemented with public PRISM spectra, to study the origin of this "V shape". By fitting a broken power law with a flexible inflection point, we find that a large fraction (20/44, nearly all spatially unresolved) of extremely red H$\alpha$ emitters at $2<z<6$ exhibit a strong change in slope, and that all strong inflections appear associated with the Balmer limit ($0.3645$ $\mu$m). Using a simple model of a reddened AGN with an unobscured scattered light component, we demonstrate that the observed "V shape" in Little Red Dots is unlikely to occur at any specific wavelength if the entire continuum is dominated by light from a power law AGN continuum. In contrast, models with an intrinsic feature at the Balmer limit, such as those that are dominated by evolved stellar populations in the rest-UV-to-optical, can produce the observed spectral shapes, provided that a reddened component picks up sufficiently redward of the break. While no model can comfortably explain the full Little Red Dot spectral energy distribution, the common inflection location suggests that it is most likely a single component that consistently dominates the rest-UV-to-optical in Little Red Dots, and that this component is associated with $T\sim10^4$ K hydrogen due to the clear preference for a break at H$_\infty$.
Forward citations
Cited by 18 Pith papers
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Misaligned or chaotic? A strong break of axial symmetry in the local LRD J1025 revealed with VLT/FORS2 spectropolarimetry
The local Little Red Dot J1025 shows a 48° polarisation-angle offset between continuum and broad H-alpha, requiring broken axial symmetry.
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(Re)solving the Complex Multiscale Morphology and V-shaped Spectral Energy Distribution of a Newly Discovered Strongly Lensed Little Red Dot in A383
Lensed LRD A383-LRD1 resolves into blue and red components ~300 pc apart whose combined light produces the canonical V-shaped SED.
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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 the Early Universe: Neutral gas Absorption in Galaxies at z > 3 from low-resolution JWST Spectroscopy
Excess Na I D absorption, interpreted as neutral-gas outflows, is detected in 20 of 811 galaxies at z>3 and rises steeply with stellar mass and quiescence.
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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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Ultrahigh-energy cosmogenic neutrino emissions in the high-redshift universe
If Little Red Dots accelerate protons to ~10 EeV, their interactions with the high-redshift CMB produce a ~50 PeV cosmogenic neutrino bump consistent with IceCube.
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The Two Orbital, Interacting Hatano-Nelson Model
Phase diagrams for a purely real spectrum are obtained in the two-particle sector of a two-chain interacting Hatano–Nelson–Hubbard model, with winding-number and Lindblad checks of boundary and open-system behavior.
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VENUS: When Red meets Blue -- A multiply imaged Little Red Dot with an apparent blue companion behind the galaxy cluster Abell 383
JWST resolves A383-LRD1 into a compact red Little Red Dot candidate and a blue companion at z≈6, magnified ~9–16× by cluster lensing.
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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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Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs). XXIV. 54 New Quasars and Candidate Obscured Quasars at $5.71 \le z \le 7.02$
Spectroscopic follow-up of the completed HSC-SSP survey yields 43 new quasars, 11 candidate obscured quasars, and 29 galaxies at z 5.71 to 7.02.
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Lonely Little Red Dots: Challenges to the AGN-nature of little red dots through their clustering and spectral energy distributions
Little red dots sit in lower-density environments than typical galaxies, and Bayesian model comparison favors non-AGN interpretations for most of them.
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Cosmic Outliers: Low-Spin Halos Explain the Abundance, Compactness, and Redshift Evolution of the Little Red Dots
Little Red Dots are explained as galaxies formed in the lowest roughly 1% of dark matter halo spin, which reproduces their abundance, compactness, and redshift distribution.
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NEXUS: A Spectroscopic Census of Broad-line AGNs and Little Red Dots at $3\lesssim z\lesssim 6$
A JWST grism survey finds 23 broad-line AGNs at z~3-6, including 15 little red dots, with host-galaxy UV emission and a tentative small-scale clustering excess.
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Evidence of violation of Case B recombination in Little Red Dots
In one of seven Little Red Dots, the broad Hδ/Hα ratio is more than 5σ below the Case B prediction, signalling a breakdown of standard recombination in very dense gas.
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The BlueDOG at Cosmic Noon: A Possible Analog to Little Red Dots?
A z=2.6 hyperluminous dust-obscured galaxy with an LRD-like SED hosts a ~10^10.2 Msun black hole, while the origin of its blue UV excess is left ambiguous.
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