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COSMOS-Web: The over-abundance and physical nature of "little red dots"--Implications for early galaxy and SMBH assembly
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abstract
JWST has revealed a population of compact and extremely red galaxies at $z>4$, which likely host active galactic nuclei (AGN). We present a sample of 434 ``little red dots'' (LRDs), selected from the 0.54 deg$^2$ COSMOS-Web survey. We fit galaxy and AGN SED models to derive redshifts and physical properties; the sample spans $z\sim5$-$9$ after removing brown dwarf contaminants. We consider two extreme physical scenarios: either LRDs are all AGN, and their continuum emission is dominated by the accretion disk, or they are all compact star-forming galaxies, and their continuum is dominated by stars. If LRDs are AGN-dominated, our sample exhibits bolometric luminosities $\sim10^{45-47}$ erg\,s$^{-1}$, spanning the gap between JWST AGN in the literature and bright, rare quasars. We derive a bolometric luminosity function (LF) $\sim100$ times the (UV-selected) quasar LF, implying a non-evolving black hole accretion density of $\sim10^{-4}$ M$_\odot$ yr$^{-1}$ Mpc$^{-3}$ from $z\sim2$-$9$. By contrast, if LRDs are dominated by star formation, we derive stellar masses $\sim10^{8.5-10}\,M_\odot$. MIRI/F770W is key to deriving accurate stellar masses; without it, we derive a mass function inconsistent with $\Lambda$CDM. The median stellar mass profile is broadly consistent with the maximal stellar mass surface densities seen in the nearby universe, though the most massive $\sim50$\% of objects exceed this limit, requiring substantial AGN contribution to the continuum. Nevertheless, stacking all available X-ray, mid-IR, far-IR/sub-mm, and radio data yields non-detections. Whether dominated by dusty AGN, compact star-formation, or both, the high masses/luminosities and remarkable abundance of LRDs implies a dominant mode of early galaxy/SMBH growth.
Forward citations
Cited by 28 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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The Cosmic Rush Hour: Rapid Formation of Bright, Massive, Disky, Star-Forming Galaxies as Signatures of Early-Universe Physics
Early dark energy in large hydrodynamic simulations reproduces JWST's excess of bright, massive, high-redshift galaxies while preserving low-redshift Lambda-CDM behavior.
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Balmer Absorption in Iron Low-Ionization Broad Absorption Line Quasars
A sample of 14 FeLoBAL quasars with Balmer absorption, eight newly identified, yields correlations between absorption opacity per velocity width and quasar luminosity/accretion, plus suggestive parallels with Little Red Dots.
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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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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 super-Eddington quasars drive clumpy circumgalactic medium and extended H$\alpha$ nebulae at $z \gtrsim 6$
In z≈6 quasar simulations, super-Eddington black-hole growth clears escape channels, ejects cold clumps that raise neutral-hydrogen covering, and powers extended Hα nebulae that shrink when the quasar is obscured.
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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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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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Discovery of Multiply Ionized Iron Emission Powered by an Active Galactic Nucleus in a z~7 Little Red Dot
The z=6.68 Little Red Dot THRILS 46403 shows a 4.5 sigma [FeVII] line, the first robust coronal-line detection among z>5 LRDs, plus narrow Balmer absorption, supporting an AGN with direct sightlines to gas at or beyon...
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Characterizing the roles of transitory obscured phases and inner torus in shaping the fractions of obscured AGN at cosmic noon
Pure evolutionary AGN light curves can only reproduce cosmic-noon obscured fractions if the visible phase is tuned by luminosity or if a long-lived torus is added; starbursts and mergers alone cannot.
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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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MEGA: Spectrophotometric SED Fitting of Little Red Dots Detected in JWST MIRI
Standard AGN and star-forming templates cannot fully reproduce the SEDs of eight MIRI-detected little red dots; dense-gas models plus extra hot dust partly work but fail on UV and narrow emission lines.
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On the Variability Features of Active Galactic Nuclei in Little Red Dots
Using the CHAR accretion-disk model, the observed non-variability of 22 little red dots is consistent with measurement errors, leaving AGN fraction and dust extinction degenerate, and ~200 sources with 2-year baseline...
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Glimmers in the Cosmic Dawn. III. On the Photometrically Determined Black Hole Mass to Stellar Mass Relation Across Cosmic Time
Photometric SED decomposition of 26 variable AGN in the HUDF yields black hole masses showing overmassive BHs at high redshift, consistent with JWST broad-line AGN.
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Beyond the Dot: an LRD-like nucleus at the Heart of an IR-Bright Galaxy and its implications for high-redshift LRDs
A z=2 LRD analog with a resolved host, plus a 99-object stacking analysis, suggests high-redshift Little Red Dots are the dimmed nuclei of extended galaxies rather than a distinct population.
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Beneath the Surface: >85% of z>5.9 QSOs in Massive Host Galaxies are UV-Faint
More than 85% of z>5.9 quasars in massive host galaxies are UV-faint, indicating that UV-bright quasars are extreme outliers in black hole mass.
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Primordial black holes in cosmological simulations: growth prospects for supermassive black holes
First cosmological hydro simulations with directly modeled PBHs show 1000 solar mass PBHs can grow to 10^4 to 10^5 solar masses by z=20 only if f_PBH is around 10^-3, not 10^-4.
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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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Physical properties of galaxies and the UV Luminosity Function from $z\sim6$ to $z\sim14$ in COSMOS-Web
A 3,099-galaxy JWST sample at z~6-14 shows a bright-end UV luminosity function excess at z~9-12 relative to evolving Schechter-function predictions, with non-evolving blue UV slopes.
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Little Red Dots from Small-Scale Primordial Black Hole Clustering
Densely clustered 30-solar-mass primordial black holes can sequentially merge into ~10^6 solar-mass seeds by redshift 6, possibly explaining JWST little red dots, with high spin from tidal torques and a two-peak gravi...
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Evolution of Size, Mass, and Density of Galaxies Since Cosmic Dawn
Under the author's CCC+TL cosmology, galaxy effective radii are larger by roughly (1+z)^0.93, reducing the inferred density and mass of early galaxies.
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Understanding the Evolution of Black Hole Accretion and Dust out to z=4 with a Deep Imaging Extragalactic Survey with PRIMA
PRIDES, a proposed deep PRIMA imaging survey over 1.6 square degrees of COSMOS, would detect tens of thousands of galaxies out to z=4 and constrain dust-obscured star formation and AGN.
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Little Red Dots as Direct-collapse Black Hole Nurseries
In the MELIORA simulations, direct-collapse black holes form mainly at z>6 during gas compaction, then enter a ~200 Myr luminous phase that could look like a Little Red Dot.
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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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