First spectroscopic variability in a z~7 LRD shows rapid changes in both narrow and broad line regions, implying direct ionization from the central source to surrounding nebular gas.
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The Missing Hard Photons of Little Red Dots: Their Incident Ionizing Spectra Resemble Massive Stars
11 Pith papers cite this work. Polarity classification is still indexing.
11
Pith papers citing it
abstract
The nature of Little Red Dots (LRDs) has largely been investigated through their continuum emission, with lines assumed to arise from a broad-line region. In this paper, we instead use recombination lines to infer the intrinsic properties of the central engine. Our analysis first reveals a tension between the ionizing properties implied from H$\alpha$ and HeII$\,\lambda$4686. The high H$\alpha$ EWs require copious H-ionizing photons, more than the bluest AGN ionizing spectra can provide. In contrast, HeII emission is marginally detected, and its low EW is, at most, consistent with the softest AGN spectra. The low HeII/H$\beta$ ($\sim10^{-2}$, $<20\times$ local AGN median) further points to an unusually soft ionizing spectrum. We extend our analysis to dense gas envelopes (``quasi-star''/``black-hole star''), and find that hydrogen recombination lines become optically thick and lose diagnostic power, but HeII remains optically thin and a robust tracer. Photoionization modeling with Cloudy rules out standard AGN accretion disk spectra. Alternative explanations include: exotic AGN with red rest-optical emission; high average optical depth ($>10$) from gas/dust; and/or soft ionizing spectra with abundant H-ionizing photons, consistent with e.g., a cold accretion disk or a composite of AGN and stars. The latter is an intriguing scenario since high hydrogen densities are highly conducive for star formation, and nuclear star clusters are found in the vicinity of local massive black holes. While previous studies have mostly focused on features dominated by the absorbing hydrogen cloud, the HeII-based diagnostic proposed here represents a crucial step toward understanding the central engine of LRDs.
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representative citing papers
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 lower limit of 7.5e-10 cMpc^-3.
Eight low-redshift Little Red Dots identified in DESI DR1 exhibit broad Balmer lines, steep decrements, compact shapes, and negligible variability, with a number density roughly 10,000 times lower than at z>4.
A z=6.64 LRD host galaxy exhibits a fast AGN-driven outflow with 5500 km/s velocities, dusty gas, and low metallicity, confirming AGN presence in these systems.
Self-gravitating disks heated by stars reach a universal optical effective temperature of 4000-4500 K independent of accretion rate, black hole mass, and viscosity, explaining Little Red Dots.
A theoretical model of a magnetized black hole envelope is developed to explain the broad emission lines and X-ray faintness observed in little red dots using co-rotating plasma clumps and limited X-ray sources.
Coevolving super-Eddington black holes and nuclear starbursts in high-redshift halos naturally generate the V-shaped UV-to-optical spectra and weak high-energy emission of little red dots.
Compact nuclear starbursts in metal-poor dwarfs produce AGN-like mid-IR colors without accretion, as revealed by JWST observations of two such galaxies.
Narrow-line diagnostics on ~20 LRDs indicate that stellar photoionization alone cannot explain the observed ratios in many objects, implying anisotropic ionizing radiation from complex gas geometry.
JWST data on LRDs and LBDs show AGN-like excitation, strong Lyα with broad components, and X-ray weakness, implying clumpy or equatorial geometries around growing black holes rather than complete gas envelopes.
A bias-controlled quasar sample of ~2000 objects demonstrates that the X-ray-to-UV luminosity relation remains constant from redshift 0.7 to 5.
citing papers explorer
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The GlimmIr: Spectroscopic Variability in a z~7 LRD Indicates Rapid Changes in Both the Narrow and Broad Line Regions
First spectroscopic variability in a z~7 LRD shows rapid changes in both narrow and broad line regions, implying direct ionization from the central source to surrounding nebular gas.
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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 lower limit of 7.5e-10 cMpc^-3.
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A new sample of Little Red Dots at $z<0.45$ in DESI DR1: Broad Balmer lines, low ionization spectrum and no variability
Eight low-redshift Little Red Dots identified in DESI DR1 exhibit broad Balmer lines, steep decrements, compact shapes, and negligible variability, with a number density roughly 10,000 times lower than at z>4.
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GLIMPSED: Direct evidence for a fast AGN-driven outflow from a z=6.64 Little Red Dot host galaxy
A z=6.64 LRD host galaxy exhibits a fast AGN-driven outflow with 5500 km/s velocities, dusty gas, and low metallicity, confirming AGN presence in these systems.
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Spectral Appearance of Self-gravitating Disks Powered by Stellar Objects: Universal Effective Temperature in the Optical Continuum and Application to Little Red Dots
Self-gravitating disks heated by stars reach a universal optical effective temperature of 4000-4500 K independent of accretion rate, black hole mass, and viscosity, explaining Little Red Dots.
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A Magnetized Black Hole Envelope Model for Little Red Dots
A theoretical model of a magnetized black hole envelope is developed to explain the broad emission lines and X-ray faintness observed in little red dots using co-rotating plasma clumps and limited X-ray sources.
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Spectral Uniformity of Little Red Dots: A Natural Outcome of Coevolving Seed Black Holes and Nascent Starbursts
Coevolving super-Eddington black holes and nuclear starbursts in high-redshift halos naturally generate the V-shaped UV-to-optical spectra and weak high-energy emission of little red dots.
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JWST Reveals Compact Nuclear Starbursts Masquerading as AGNs in Metal-Poor Dwarfs: Where Are the Accreting Intermediate-Mass Black Holes?
Compact nuclear starbursts in metal-poor dwarfs produce AGN-like mid-IR colors without accretion, as revealed by JWST observations of two such galaxies.
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Constraints on the Gas Geometry Surrounding Little Red Dots through Narrow-Line Diagnostics
Narrow-line diagnostics on ~20 LRDs indicate that stellar photoionization alone cannot explain the observed ratios in many objects, implying anisotropic ionizing radiation from complex gas geometry.
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Little Red and Blue Dots: AGN-excited narrow lines, Lyman-$\alpha$ emission, and resemblance to standard quasars
JWST data on LRDs and LBDs show AGN-like excitation, strong Lyα with broad components, and X-ray weakness, implying clumpy or equatorial geometries around growing black holes rather than complete gas envelopes.
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The X-ray-to-UV relation does not evolve in homogeneous quasar samples
A bias-controlled quasar sample of ~2000 objects demonstrates that the X-ray-to-UV luminosity relation remains constant from redshift 0.7 to 5.