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Little Red Dots: Rapidly Growing Black Holes Reddened by Extended Dusty Flows
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abstract
The James Webb Space Telescope (JWST) observations have revolutionized extragalactic research, particularly with the discovery of little red dots (LRD), which we propose are dust-reddened broad-line active galactic nuclei (AGNs). Their unique v-shape spectral feature observed through JWST/NIRCam challenges us to discern the relative contributions of the galaxy and AGN. We study a spectral energy distribution (SED) model for LRDs from rest-frame UV to infrared bands. We hypothesize that the incident radiation from an AGN, characterized by a typical SED, is embedded in an extended dusty medium with an extinction law similar to those seen in dense regions such as Orion Nebula or certain AGN environments. The UV-optical spectrum is described by dust-attenuated AGN emission, featuring a red optical continuum at $\lambda>4000$ A and a flat UV spectral shape established through a gray extinction curve at $\lambda<3000$ A, due to the absence of small-size grains. There is no need for additional stellar emission or AGN scattered light. In the infrared, the SED is shaped by an extended dust and gas distribution ($\gamma<1$; $\rho\propto r^{-\gamma}$) with a characteristic gas density of $\simeq 10-10^3~{\rm cm}^{-3}$, which allows relatively cool dust temperatures to dominate the radiation, thereby shifting the energy peak from near- to mid-infrared bands. This model, unlike the typical AGN hot torus models, can produce an infrared SED flattening that is consistent with LRD observations through JWST MIRI. Such a density structure can arise from the coexistence of inflows and outflows during the early assembly of galactic nuclei. This might be the reason why LRDs emerge preferentially in the high-redshift universe younger than one billion years.
Forward citations
Cited by 8 Pith papers
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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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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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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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Lack of Rest-frame UV Variability in Little Red Dots Based on HST and JWST Observations
No little red dot in a 21-source sample shows 3-sigma rest-frame UV variability over 6 to 11 years, giving a model-dependent upper limit of about 30% on AGN contribution to their UV light.
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Weakness of X-rays and Variability in High-redshift AGNs with Super-Eddington Accretion
A super-Eddington disk with an optically thick warm corona reproduces the X-ray weakness and low UV/optical variability of high-redshift JWST AGNs.
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A Comprehensive Photometric Selection of `Little Red Dots' in MIRI Fields: An IR-Bright LRD at $z=3.1386$ with Warm Dust Emission
A new photometric selection recovers 248 Little Red Dots across JWST fields, increasing the known sample by about 1.7x, and reveals warm dust emission in one low-redshift LRD.
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Exploring the Nature of Little Red Dots: Constraints on AGN and Stellar Contributions from PRIMER MIRI Imaging
MIRI photometry of 95 little red dots shows that neither a pure stellar nor a pure AGN model can easily explain their light, pointing to mixed contributions or exotic physics.
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Episodic super-Eddington accretion as a clue to Overmassive Black Holes in the early Universe
Short episodes of super-Eddington accretion triggered by major mergers can produce the overmassive black holes seen by JWST in the early Universe.
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