Little Red Dots

The little red dot J1148-18404 (center) in false color, located about 12.6 billion light-years away in the constellation Ursa Major.
Unsolved problem in astronomy
What is the nature of Little Red Dots?
More unsolved problems in astronomy

Little red dots (LRDs) are a class of small, red-tinted astronomical objects with unexpected characteristics observed using the James Webb Space Telescope (JWST). They appear to have existed between 0.6 and 1.6 billion years after the Big Bang (13.2 to 12.2 billion years ago), with a majority found around 600 million years post-Big Bang. As of 2025, over 300 little red dots have been observed.

Various efforts to explain these observations rely on analogies to existing stars or on models. The original reports identified LRDs as a type of early active galactic nucleus (AGN) containing a supermassive black hole. However, while this explains their age and appearance, they do not have the same characteristics as known AGNs. For example, they do not appear to emit X-rays, have a flattened rather than steeply rising infrared spectrum, and display very little variability between themselves. Other theories proposed include that LRDs are supermassive stars, luminous precursors to supermassive black holes or are that they are quasi-stars (black hole stars).

Models

Several models have been proposed to explain the observed properties of little red dots. The shape of the ultraviolet spectrum can be explained by the scattered AGN light or by the gray dust extinction law. The spectral energy distribution (SED) for many LRDs can be modeled using clumps of massive, hot OB stars or active galactic nuclei within dense dusty regions which absorb and remit the energy of the stars.

As active galactic nuclei

False-color stamps of 6 Little Red Dots with redshift given

Little red dots were first reported as abundant, faint active galactic nuclei (AGN) found by seeking broad Balmer line emitters. Subsequent surveys confirmed their abundance and their relative faint UV spectrum. Active galactic nuclei are defined as small regions in the centers of galaxies that emit copious amounts of energy in the form of bright jets and winds. One property of LRDs explained by the AGN theory is the red color of the galaxies themselves. Astrophysicists have determined that the distinct color can be attributed to the massive amounts of gas, dust, and electromagnetic energy that surrounds the AGN and supermassive black hole. This region is also known as the accretion disk.

A team using the Webb telescope targeted LRDs in the 'Red Unknowns: Bright Infrared Extragalactic Survey', observing rapid gas orbits of roughly 900 km/s—a strong indicator of black hole accretion.

More direct evidence for the black-hole interpretation came in 2026 from Abell 2744-QSO1. Spatially resolved observations showed a rotation curve inconsistent with a nuclear star cluster but consistent with Keplerian rotation around a point mass of approximately 50 million solar masses. The dynamical mass agreed with previous virial black-hole mass estimates, demonstrating that such estimates are applicable to this prototypical LRD.

On the other hand, LRDs also exhibit properties that are difficult to explain within the AGN scenario. For example, they have a flat infrared spectrum, and little X-ray emission has been detected. LRDs also show very weak time variability, while high variability is often seen in AGN observation.

As supermassive population III stars

Another theory is that the little red dots are supermassive stars, luminous precursors to supermassive black holes. These non-metallic primordial stars—also known as population III stars— contain perhaps a million times the Sun's mass. Theoretical modelling of such stars closely match the spectrum features and luminosity of LRDs, including the presence of a "strong, broad Hβ emission line alongside other Balmer lines in absorption", and in particular the photosphere of such a star would cause the V-shaped Balmer break seen in LRDs.

As black hole stars

Another theory proposes that LRDs are quasi-stars or similar objects, consisting of a black hole surrounded by a gaseous envelope. Models suggest that the red color and spectrum of LRD MoM-BH*-1 is a result of light scattering from dense ionized gas surrounding a black hole.

Observed properties

Research has shown that LRDs do not commonly exist at lower redshifts. One possible reason for this observation is "inside-out growth." When a galaxy evolves and expands outward from its nucleus at lower redshifts, a decreasing amount of gas is deposited near the accreting black hole. Thus, the black hole sheds its outer gas layers, becomes bluer, and is no longer categorized as an LRD.

Most are extremely compact, averaging around 2% of the radius of the Milky Way. A typical LRD has a radius no greater than 500 light-years, though many have radii smaller than 150 light-years.

From a sample of 99 little red dots analyzed for morphology, 69 were predominantly compact without extended components, with the other 30 with more complex morphologies. Of these complex galaxies, 50% showed multiple associated components, and the rest showed highly asymmetric structures, with indications of a composite nature. It is hypothesized from this analysis that LRDs may be a product of galaxy interactions and mergers, with potential evidence to suggest early stages of galaxy and black hole growth. These suspected young black holes are among the smallest recorded, at 105 – 107 solar masses.

Likely local analogues of LRDs were discovered in a sample of green pea galaxies (GP). These are broad-line AGN-hosting green peas (BLGP) with V-shaped rest-frame UV-to-optical SED. Seven such V-shaped BLGPs were identified from a sample size of 2,190. These V-shaped BLGPs host over-massive black holes.

RUBIES

RUBIES footprint of 18 NIRSpec/MSA pointings in the UDS and EGS fields. Purple pointings correspond to the first half of observations in January to March 2024. Background images show the NIRCam F444W image mosaics, primarily constructed from public imaging of the CEERS and PRIMER surveys. The outline of the PRIMER MIRI imaging footprint for the UDS is shown in pink.

RUBIES, the "Red Unknowns: Bright Infrared Extragalactic Survey", is a JWST program led by Anna de Graaff and Gabriel Brammer that observed ~300 "very red sources" in the Ultra Deep Survey (UDS) and Extended Groth Strip (EGS) fields. Research conducted in association with RUBIES program has found that sources of a selected group (2 < z < 5) have a majority of massive quiescent galaxies, which is 10 times the original estimated value. The program has also found that observed LRDs have much lower levels of hot and cold gas than models would suggest, pointing away from the possibilities of AGNs or star-forming galaxies, although this is still debated.

Notable LRDs

The Cliff (RUBIES-UDS-154183)

NIRSpec/PRISM spectrum of The Cliff. The orange points show NIRCam and MIRI photometry.

The Cliff is an LRD with a prominent Balmer break, discovered via JWST's RUBIES program. Detailed spectroscopic observations suggest that The Cliff might be a black hole star.

CAPERS-LRD-z9

CAPERS-LRD-z9 is a little red dot confirmed to be a broad-line active galactic nucleus (BLAGN) with the redshift z = 9.288. It is the highest redshift AGN known. CAPERS-LRD-z9 exhibits a prominent Balmer break and "provides strong evidence in support of the 'dense-gas-enshrouded AGN'" explanation. These unusual Balmer jumps are notable properties of LRDs, causing difficulties in observations and spectroscopic analysis. Some LRDs exhibit a symmetrical density distribution of electron scattering, thus a gaseous envelope absorbing nebular spectra is a credible possibility.

A2744–45924

The LRD A2744–45924 is located in the Abell 2744 field, and is the most optically luminous LRD found by JWST.

RUBIES-BLAGN-1

RUBIES-BLAGN-1 is an LRD which is "an unusually bright LRD (zspec = 3.1) observed as part of the RUBIES program. This LRD exhibits broad emission lines ( FWHM ~ 4000 km s−1), a blue UV continuum, a clear Balmer break, and a red continuum sampled out to rest-frame 4 μm with MIRI."

J1007_AGN

The LRD J1007_AGN has a redshift z = 7.3, and is "embedded in an overdensity of eight nearby galaxies".

Abell 2744-QSO1

JWST NIRCam image of Abell 2744-QSO1. The object appears three times (QSO1A, QSO1B and QSO1C) due to gravitational lensing by the galaxy cluster Abell 2744.

Abell 2744-QSO1 is a strongly lensed little red dot at a redshift of z = 7.04. It was described as a "naked" black hole, because very few stars are in its vicinity. Observations of its gas kinematics yielded a direct dynamical measurement of its central black hole, with the observed rotation curve consistent with Keplerian motion around a point mass of approximately 50 million solar masses. The measurement agreed with earlier virial black-hole mass estimates. The inferred ratio of black-hole mass to stellar mass was greater than two, inconsistent with a substantial stellar host galaxy. The proposed alternative is a massive black-hole seed observed during an early phase of accretion.

MoM-BH*-1

MoM-BH*-1 is a prominent LRD described as "a higher-redshift analogue of The Cliff". MoM-BH*-1 outshines the galaxy it hosts and models suggest its red color derives from light emitted as gas falls into a black hole scattering off surrounding gas on the way to telescopes. A team of astronomers led by Rohan P. Naidu has suggested light from MoM-BH*-1 could be explained in terms of an "enshrouded black hole".

References

  1. ↑ Boyle, Rebecca (October 9, 2024). "The 'Beautiful Confusion' of the First Billion Years Comes Into View". Quanta Magazine. Archived from the original on October 17, 2024. Retrieved October 18, 2024.
  2. 1 2 Pacucci, Fabio (September 8, 2024). "Hidden, compact galaxies in the distant universe—searching for the secrets behind the little red dots". The Conversation. Phys.org. Archived from the original on April 10, 2025. Retrieved March 15, 2025.
  3. 1 2 3 "Newfound Galaxy Class May Indicate Early Black Hole Growth, Webb Finds". Webb. January 14, 2025. Archived from the original on January 20, 2025. Retrieved February 3, 2025.
  4. 1 2 3 De Graaff, Anna; Brammer, Gabriel; Weibel, Andrea; et al. (2025). "RUBIES: A complete census of the bright and red distant Universe with JWST/NIRSpec". Astronomy & Astrophysics. 697: A189. arXiv:2409.05948. Bibcode:2025A&A...697A.189D. doi:10.1051/0004-6361/202452186. Archived from the original on November 15, 2025. Retrieved October 22, 2025.
  5. 1 2 "Are the JWST's Little Red Dots Actually Supermassive Black Hole Seeds?". July 21, 2025. Archived from the original on July 22, 2025. Retrieved July 22, 2025.
  6. 1 2 Kocevski, Dale D.; Onoue, Masafusa; Inayoshi, Kohei; et al. (September 1, 2023). "Hidden Little Monsters: Spectroscopic Identification of Low-mass, Broad-line AGNs at z > 5 with CEERS". The Astrophysical Journal Letters. 954 (1): L4. arXiv:2302.00012. Bibcode:2023ApJ...954L...4K. doi:10.3847/2041-8213/ace5a0. ISSN 2041-8205.
  7. 1 2 Labbe, Ivo; Greene, Jenny E.; Bezanson, Rachel; et al. (2023). "UNCOVER: Candidate Red Active Galactic Nuclei at 3 < z < 7 with JWST and ALMA". The Astrophysical Journal. 978 (1): 92. arXiv:2306.07320. Bibcode:2025ApJ...978...92L. doi:10.3847/1538-4357/ad3551.
  8. 1 2 Li, Zhengrong; Inayoshi, Kohei; Chen, Kejian; Ichikawa, Kohei; Ho, Luis C. (2025). "Little Red Dots: Rapidly Growing Black Holes Reddened by Extended Dusty Flows". The Astrophysical Journal. 980 (1): 36. arXiv:2407.10760. Bibcode:2025ApJ...980...36L. doi:10.3847/1538-4357/ada5fb.
  9. ↑ Pérez-González, Pablo G.; Barro, Guillermo; Rieke, George H.; et al. (June 1, 2024). "What Is the Nature of Little Red Dots and what Is Not, MIRI SMILES Edition". The Astrophysical Journal. 968 (1): 4. arXiv:2401.08782. Bibcode:2024ApJ...968....4P. doi:10.3847/1538-4357/ad38bb. ISSN 0004-637X.
  10. ↑ Matthee, Jorryt; Naidu, Rohan P.; Brammer, Gabriel; et al. (March 7, 2024). "Little Red Dots: An Abundant Population of Faint Active Galactic Nuclei at z ~ 5 Revealed by the EIGER and FRESCO JWST Surveys". The Astrophysical Journal. 963 (2): 129. arXiv:2306.05448. Bibcode:2024ApJ...963..129M. doi:10.3847/1538-4357/ad2345. ISSN 0004-637X.
  11. ↑ Greene, Jenny E.; Labbe, Ivo; Goulding, Andy D.; et al. (March 1, 2024). "UNCOVER Spectroscopy Confirms the Surprising Ubiquity of Active Galactic Nuclei in Red Sources at z > 5". The Astrophysical Journal. 964 (1): 39. arXiv:2309.05714. Bibcode:2024ApJ...964...39G. doi:10.3847/1538-4357/ad1e5f. ISSN 0004-637X.
  12. ↑ "What Are Active Galactic Nuclei?". Webb. Archived from the original on June 10, 2024. Retrieved January 30, 2025.
  13. ↑ "Active Galactic Nucleus". ESA/Hubble. Archived from the original on April 15, 2025. Retrieved January 30, 2025.
  14. ↑ Lea, Robert (January 27, 2025). "Supermassive black holes in 'little red dot' galaxies are 1,000 times larger than they should be, and astronomers don't know why". Space.com. Archived from the original on January 29, 2025. Retrieved January 30, 2025.
  15. ↑ Juodžbalis, Ignas; Marconcini, Cosimo; D'Eugenio, Francesco; Maiolino, Roberto (27 May 2026). "A direct black-hole mass measurement in a little red dot at high redshift". Nature. 653 (8116): 1017–1021. doi:10.1038/s41586-026-10579-4. PMC 13215880. PMID 42203931.
  16. ↑ Williams, Christina C.; Alberts, Stacey; Ji, Zhiyuan; et al. (June 1, 2024). "The Galaxies Missed by Hubble and ALMA: The Contribution of Extremely Red Galaxies to the Cosmic Census at 3 < z < 8". The Astrophysical Journal. 968 (1): 34. arXiv:2311.07483. Bibcode:2024ApJ...968...34W. doi:10.3847/1538-4357/ad3f17. ISSN 0004-637X.
  17. ↑ Ananna, Tonima Tasnim; Bogdán, Ákos; Kovács, Orsolya E.; et al. (July 1, 2024). "X-Ray View of Little Red Dots: Do They Host Supermassive Black Holes?". The Astrophysical Journal Letters. 969 (1): L18. arXiv:2404.19010. Bibcode:2024ApJ...969L..18A. doi:10.3847/2041-8213/ad5669. ISSN 2041-8205.
  18. ↑ Yue, Minghao; Eilers, Anna-Christina; Ananna, Tonima Tasnim; et al. (October 1, 2024). "Stacking X-Ray Observations of "Little Red Dots": Implications for Their Active Galactic Nucleus Properties". The Astrophysical Journal Letters. 974 (2): L26. arXiv:2404.13290. Bibcode:2024ApJ...974L..26Y. doi:10.3847/2041-8213/ad7eba. ISSN 2041-8205.
  19. ↑ Kokubo, Mitsuru; Harikane, Yuichi (2025). "Challenging the Active Galactic Nucleus Scenario for JWST/NIRSpec Little Red Dot and Non–Little Red Dot Broad H α Emitters in Light of Nondetection of NIRCam Photometric Variability and X-Ray". The Astrophysical Journal. 995 (1): 24. arXiv:2407.04777. Bibcode:2025ApJ...995...24K. doi:10.3847/1538-4357/ae119e.
  20. ↑ Nandal, Devesh; Loeb, Abraham (2025). "Supermassive Stars Match the Spectral Signatures of JWST's Little Red Dots". The Astrophysical Journal. 998 (1): 124. arXiv:2507.12618. Bibcode:2026ApJ...998..124N. doi:10.3847/1538-4357/ae32f3.
  21. ↑ Begelman, Mitchell C.; Dexter, Jason (2026). "Little Red Dots as Late-stage Quasi-stars". The Astrophysical Journal. 996 (1): 48. arXiv:2507.09085. Bibcode:2026ApJ...996...48B. doi:10.3847/1538-4357/ae274a.
  22. ↑ Kido, Daisaburo; Ioka, Kunihito; Hotokezaka, Kenta; et al. (2025). "Black Hole Envelopes in Little Red Dots". Monthly Notices of the Royal Astronomical Society. 544 (4): 3407–3416. arXiv:2505.06965. Bibcode:2025MNRAS.544.3407K. doi:10.1093/mnras/staf1898.
  23. ↑ "Little Red Dots: Stars or Black Holes?". NASA Space News. September 9, 2024. Archived from the original on November 27, 2024. Retrieved October 18, 2024.
  24. 1 2 Schleicher, Dominik; Herrera-Camus, Rodrigo (August 12, 2026). "'Little red dot' gets its colour from gas — not dust". Nature. 656 (8127): 301–302. Bibcode:2026Natur.656..301S. doi:10.1038/d41586-026-02290-1. PMID 42587121.
  25. ↑ Pacucci, Fabio. "JWST's 'Little Red Dots' Offer Astronomers the Universe's Weirdest Puzzle". Scientific American. Archived from the original on March 4, 2025. Retrieved February 19, 2025.
  26. ↑ Rinaldi, P.; Bonaventura, N.; Rieke, G. H.; et al. (October 10, 2025). "Not Just a Dot: The Complex UV Morphology and Underlying Properties of Little Red Dots". The Astrophysical Journal. 992 (1): 71. arXiv:2411.14383. Bibcode:2025ApJ...992...71R. doi:10.3847/1538-4357/adfa10. hdl:11370/df23ecc6-c5d0-470e-b470-104468e02e06. ISSN 0004-637X.
  27. 1 2 Rusakov, V.; Watson, D.; Nikopoulos, G. P.; et al. (January 14, 2026). "Little red dots as young supermassive black holes in dense ionized cocoons". Nature. 649 (8097): 574–579. arXiv:2503.16595. Bibcode:2026Natur.649..574R. doi:10.1038/s41586-025-09900-4. ISSN 1476-4687. PMC 12804088. PMID 41535486.
  28. 1 2 Lin, Ruqiu; Zheng, Zhen-Ya; Jiang, Chunyan; Yuan, Fang-Ting; Ho, Luis C.; Wang, Junxian; Jiang, Linhua; Rhoads, James E.; Malhotra, Sangeeta; Barrientos, L. Felipe; Wold, Isak; Infante, Leopoldo; Zhu, Shuairu; Ji, Xiang; Fu, Xiaodan (2025). "Discovery of Local Analogs to JWST's Little Red Dots". The Astrophysical Journal Letters. 980 (2): L34. arXiv:2412.08396v1. Bibcode:2025ApJ...980L..34L. doi:10.3847/2041-8213/adaaf1.
  29. ↑ "RUBIES | Anna de Graaff". annadeg.github.io. Archived from the original on October 19, 2025. Retrieved October 24, 2025.
  30. 1 2 Wang 王, Bingjie 冰洁; De Graaff, Anna; Davies, Rebecca L.; et al. (2025). "RUBIES: JWST/NIRSpec Confirmation of an Infrared-luminous, Broad-line Little Red Dot with an Ionized Outflow". The Astrophysical Journal. 984 (2): 121. arXiv:2403.02304. Bibcode:2025ApJ...984..121W. doi:10.3847/1538-4357/adc1ca.
  31. ↑ Zhang, Yunchong; de Graaff, Anna; Setton, David J.; Price, Sedona H.; Bezanson, Rachel; del P. Lagos, Claudia; Cutler, Sam E.; McConachie, Ian; Cleri, Nikko J.; Cooper, Olivia R.; Gottumukkala, Rashmi; Greene, Jenny E.; Hirschmann, Michaela; Khullar, Gourav; Labbe, Ivo (January 27, 2026). "RUBIES Spectroscopically Confirms the High Number Density of Quiescent Galaxies from 2 < z < 5". The Astrophysical Journal. 997 (2): 252. arXiv:2508.08577. Bibcode:2026ApJ...997..252Z. doi:10.3847/1538-4357/ae24e1. ISSN 0004-637X.
  32. ↑ Setton, David J.; Greene, Jenny E.; Spilker, Justin S.; et al. (September 12, 2025). "A Confirmed Deficit of Hot and Cold Dust Emission in the Most Luminous Little Red Dots". The Astrophysical Journal Letters. 991 (1): L10. arXiv:2503.02059. Bibcode:2025ApJ...991L..10S. doi:10.3847/2041-8213/ade78b. hdl:11370/2e0b28ba-601d-48a5-b893-b75213d07a8f. ISSN 2041-8205.
  33. 1 2 3 de Graaff, Anna; Rix, Hans-Walter; Naidu, Rohan P.; et al. (September 1, 2025). "A remarkable ruby: Absorption in dense gas, rather than evolved stars, drives the extreme Balmer break of a little red dot at z = 3.5". Astronomy & Astrophysics. 701: A168. arXiv:2503.16600. Bibcode:2025A&A...701A.168D. doi:10.1051/0004-6361/202554681. ISSN 0004-6361. Archived from the original on October 16, 2025. Retrieved October 17, 2025.
  34. ↑ Ahart, Jenna (October 15, 2025). "Mysterious cosmic 'dots' are baffling astronomers. What are they?". Nature. 646 (8086): 779–780. doi:10.1038/d41586-025-03352-6. ISSN 1476-4687. PMID 41094054.
  35. ↑ Taylor, Anthony J.; Kokorev, Vasily; Kocevski, Dale D.; et al. (August 10, 2025). "CAPERS-LRD-z9: A Gas-enshrouded Little Red Dot Hosting a Broad-line Active Galactic Nucleus at z = 9.288". The Astrophysical Journal Letters. 989 (1): L7. arXiv:2505.04609. Bibcode:2025ApJ...989L...7T. doi:10.3847/2041-8213/ade789.
  36. ↑ Labbe, Ivo; et al. (2024). "An unambiguous AGN and a Balmer break in an Ultraluminous Little Red Dot at z=4.47 from Ultradeep UNCOVER and All the Little Things Spectroscopy". arXiv:2412.04557 [astro-ph.GA].
  37. ↑ Schindler, Jan-Torge; Hennawi, Joseph F.; Davies, Frederick B.; et al. (2025). "A little red dot at z = 7.3 within a large galaxy overdensity". Nature Astronomy. 9 (11): 1732–1744. arXiv:2411.11534. Bibcode:2025NatAs...9.1732S. doi:10.1038/s41550-025-02660-1.
  38. ↑ Wood, Charlie (September 12, 2025). "A Single, 'Naked' Black Hole Rewrites the History of the Universe". Archived from the original on May 4, 2026. Retrieved September 14, 2025.
  39. ↑ Juodžbalis, Ignas; Marconcini, Cosimo; D'Eugenio, Francesco; Maiolino, Roberto (27 May 2026). "A direct black-hole mass measurement in a little red dot at high redshift". Nature. 653 (8116): 1017–1021. doi:10.1038/s41586-026-10579-4. PMC 13215880. PMID 42203931.
  40. ↑ Naidu, Rohan P.; et al. (August 12, 2026). "A gas-enshrouded and gas-reddened black hole at cosmic dawn". Nature. 656 (8127). Springer Science and Business Media LLC: 329–333. arXiv:2503.16596. Bibcode:2026Natur.656..329N. doi:10.1038/s41586-026-10846-4. ISSN 0028-0836. PMC 13468124. PMID 42587117.
  41. ↑ Sample, Ian (August 12, 2026). "Astronomers discover a new kind of cosmic object – a black hole 'star'". The Guardian. Retrieved August 12, 2026.

Further reading