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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 6 Pith papers
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Little Red Dots as Hidden Neutrino Sources
Little Red Dots can contribute ~30% of the diffuse neutrino background at TeV-sub-PeV energies through photomeson production in black hole envelopes, with modified flavor ratios at higher energies.
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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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Little Red Dots at z~2 in EIGER reveal a gentle decline with respect to their peak number density at z~5
Five LRDs at z≈2 yield number density ≈7×10^{-6} cMpc^{-3}, confirming a decline from the z≈5 peak but gentler than prior photometric estimates.
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Grain-size evolution and rapid dust growth in high-redshift galaxies
A multiphase ISM grain-size model with low supernova dust yield reproduces observed dust-to-stellar mass ratios and UV luminosity functions at z=7-12 by letting small grains seed rapid metal accretion.
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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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