Little red dots

Galaxies
AGN
Author
Affiliation

Hollis Akins

UT Austin

Published

April 2, 2025

Modified

October 1, 2025

“Little Red Dots” (LRDs) are a recently discovered population of faint, red, compact sources with broad emission lines identified in deep JWST imaging and spectroscopy. First identified in NIRCam grism data, these objects appear to be low-mass active galactic nuclei (AGN) at high redshifts (\(z \sim 4\)-\(9\)), providing new insights into black hole growth and evolution in the early universe. Their abundance may challenge previous models of black hole seeding and growth, suggesting more efficient accretion mechanisms in the early universe.

History

The term “Little red dot” was coined by Matthee et al. (2024) alongside their identification of numerous broad-line AGN in NIRCam grism data from the FRESCO and EIGER surveys. Arguably, however, the LRDs were known previously, reported by Labbé et al. (2023) as “double break” galaxies from early imaging from the CEERS survey. Similar to Lyman break galaxies, these were identified with a color-color selection to capture objects with strong Lyman breaks and Balmer breaks.

Key observational characteristics include: - Extremely compact morphology, often indistinguishable from a point source - Red colors in the rest-frame optical (typically \(F277W - F444W > 1\)) - Broad H\(\alpha\) and/or H\(\beta\) emission lines with typical FWHM of 1000-2500 km/s

Examples

Kokorev et al. (2023) report the spectroscopic identification of an LRD at \(z=8.50\) from the UNCOVER survey.

Selection

Selection of LRDs is some

Not all BLAGN are LRDs, and not all LRDs are BLAGN, but it is not yet established how well the two populations overlap.

Spectral energy distribution

The LRD SED has been the subject of a lot of work. It is characterized by a composite SED shape that is blue in the rest-frame UV (e.g., with UV slope \(\beta \lesssim -1\)) but red in the rest-frame optical (\(\beta_{\rm opt} \gtrsim 0\)). Setton et al. (2024) stacked RUBIES spectra and found that this turnover typically occurs at the Balmer limit.

Example SED fits to LRDs, from Kocevski et al. (2024). These objects were selected based on their blue UV+red optical SEDs as indicated by the blue/red lines.

The LRDs lack many AGN signatures typically observed at lower-redshift and in more normal AGN, such as X-ray emission and mid-infrared emission from the dust torus.

Stacked SED from the X-ray to the radio, from Akins et al. (2024).

Demographics

Recent studies have measured the number density of LRDs to be approximately \(n \sim 10^{-5}\) Mpc\(^{-3}\) at \(z \sim 4\)-\(6\), higher than expected based on extrapolations from luminous quasar populations (Greene et al. 2024; Akins et al. 2024; Kokorev et al. 2024; Kocevski et al. 2024).

The LRDs don’t seem to exist in large numbers below \(z<3\) (Kocevski et al. 2024), though its unclear if this is a selection effect.

Theoretical Implications

Black Hole Seeding and Growth

The discovery of numerous low-mass AGN at high redshifts has implications for black hole formation theories:

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References

Akins, Hollis B., Caitlin M. Casey, Erini Lambrides, Natalie Allen, Irham T. Andika, Malte Brinch, Jaclyn B. Champagne, et al. 2024. “COSMOS-Web: The over-abundance and physical nature of ‘little red dots’–Implications for early galaxy and SMBH assembly.” arXiv e-Prints, June, arXiv:2406.10341. https://doi.org/10.48550/arXiv.2406.10341.
Greene, Jenny E., Ivo Labbe, Andy D. Goulding, Lukas J. Furtak, Iryna Chemerynska, Vasily Kokorev, Pratika Dayal, et al. 2024. “UNCOVER Spectroscopy Confirms the Surprising Ubiquity of Active Galactic Nuclei in Red Sources at z > 5” 964 (1): 39. https://doi.org/10.3847/1538-4357/ad1e5f.
Kocevski, Dale D., Steven L. Finkelstein, Guillermo Barro, Anthony J. Taylor, Antonello Calabrò, Brivael Laloux, Johannes Buchner, et al. 2024. “The Rise of Faint, Red AGN at \(z>4\): A Sample of Little Red Dots in the JWST Extragalactic Legacy Fields.” arXiv e-Prints, April, arXiv:2404.03576. https://doi.org/10.48550/arXiv.2404.03576.
Kokorev, Vasily, Karina I. Caputi, Jenny E. Greene, Pratika Dayal, Maxime Trebitsch, Sam E. Cutler, Seiji Fujimoto, et al. 2024. “A Census of Photometrically Selected Little Red Dots at 4 < z < 9 in JWST Blank Fields” 968 (1): 38. https://doi.org/10.3847/1538-4357/ad4265.
Kokorev, Vasily, Seiji Fujimoto, Ivo Labbe, Jenny E. Greene, Rachel Bezanson, Pratika Dayal, Erica J. Nelson, et al. 2023. “UNCOVER: A NIRSpec Identification of a Broad-line AGN at z = 8.50” 957 (1): L7. https://doi.org/10.3847/2041-8213/ad037a.
Labbé, Ivo, Pieter van Dokkum, Erica Nelson, Rachel Bezanson, Katherine A. Suess, Joel Leja, Gabriel Brammer, et al. 2023. “A population of red candidate massive galaxies 600 Myr after the Big Bang” 616 (7956): 266–69. https://doi.org/10.1038/s41586-023-05786-2.
Lambrides, Erini, Kristen Garofali, Rebecca Larson, Andrew Ptak, Marco Chiaberge, Arianna S. Long, Taylor A. Hutchison, et al. 2024. “The Case for Super-Eddington Accretion: Connecting Weak X-ray and UV Line Emission in JWST Broad-Line AGN During the First Gyr of Cosmic Time.” arXiv e-Prints, September, arXiv:2409.13047. https://doi.org/10.48550/arXiv.2409.13047.
Matthee, Jorryt, Rohan P. Naidu, Gabriel Brammer, John Chisholm, Anna-Christina Eilers, Andy Goulding, Jenny Greene, et al. 2024. “Little Red Dots: An Abundant Population of Faint Active Galactic Nuclei at z \(\sim\) 5 Revealed by the EIGER and FRESCO JWST Surveys” 963 (2): 129. https://doi.org/10.3847/1538-4357/ad2345.
Pacucci, Fabio, and Ramesh Narayan. 2024. “Mildly Super-Eddington Accretion onto Slowly Spinning Black Holes Explains the X-Ray Weakness of the Little Red Dots” 976 (1): 96. https://doi.org/10.3847/1538-4357/ad84f7.
Setton, David J., Jenny E. Greene, Anna de Graaff, Yilun Ma, Joel Leja, Jorryt Matthee, Rachel Bezanson, et al. 2024. “Little Red Dots at an Inflection Point: Ubiquitous ‘V-Shaped’ Turnover Consistently Occurs at the Balmer Limit.” arXiv e-Prints, November, arXiv:2411.03424. https://doi.org/10.48550/arXiv.2411.03424.