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Little Red Dots as the Very First Activity of Black Hole Growth
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abstract
The James Webb Space Telescope has detected massive black holes (BHs) with masses of $\sim 10^{6-8}~M_\odot$ within the first billion years of the universe. One of the remarkable findings is the identification of "Little Red Dots" (LRDs), a unique class of active galactic nuclei (AGNs) with distinct characteristics representing a key phase in the formation and growth of early BHs. Here, we analyze the occurrence rate of LRDs, which emerge around redshifts $z \sim 6-8$ and sharply decline at $z < 4$. We find that this trend follows a log-normal distribution, commonly observed in phenomena driven by stochastic and random factors. We propose a hypothesis that the first one or two AGN events associated with newly-formed seed BHs are observed as LRDs and their unique features fade in the subsequent episodes. This naturally explains the cosmic evolution of AGN abundance over $0 < z < 5$, which follows $\propto (1+z)^{-5/2}$ due to the cumulative effect of recurring AGN activity. The unique characteristics of LRDs are likely linked to the dense gas environments around the seed BHs, which create strong absorption features in the broad-line emission and enable super-Eddington accretion bursts, ultimately yielding the observed overmassive nature of BHs compared to the local relationship. An analytical expression for the redshift evolution of LRD abundance is provided for direct comparison with future observational constraints.
Forward citations
Cited by 8 Pith papers
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A direct black hole mass measurement in a Little Red Dot at the Epoch of Reionization
A lensed Little Red Dot at z=7.04 shows Keplerian rotation around a central point mass, giving the first direct dynamical black hole mass measurement in the epoch of reionization.
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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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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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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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The Emergence and Ionizing Feedback of Pop III.1 Stars as Progenitors for Supermassive Black Holes
Radiation-hydro simulations show Pop III.1 stars ionize bubbles of about 1 cMpc, which sets an isolation distance that yields n_SMBH ~ 10^-1 cMpc^-3 by z=14.
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Little Red reionization factories
LRDs may drive cosmic reionization: tidal fields are said to funnel intergalactic hydrogen into colliding streams at LRD sites, igniting starbursts that ionize the gas.
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How similar are narrow-line Seyfert 1 galaxies and high-z type 1 AGN?
NLS1s and high-z JWST AGN share low black hole mass, high Eddington ratio, and narrow broad lines, so NLS1s are useful analogs, though host-to-BH ratios and line profiles differ.
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