Little red dot (astronomical object)
Poorly understood class of astronomical objects
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Little red dots (LRDs) are a class of small, red-tinted astronomical objects discovered by the James Webb Space Telescope (JWST).[1][2][3][4] Their discovery was announced in March 2024, and they are poorly understood due to limited data collection.[5] 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.[2][6] As of 2025, 341 LRDs have been identified with JWST.[7]
The current leading theory is that the LRDs are a form of primordial galaxy,[8] and 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.[9]
In July 2025 it was proposed that LRDs were instead supermassive non-metallic primordial stars—also known as population III stars—of perhaps a million solar masses, seen in the last few thousands of years of their lifetimes.[8] Theoretical modelling of such stars appeared to 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.[8] The authors further hypothesized that such stars were progenitors of supermassive black holes, also explaining the early development of the latter objects.[8]
Other theories are that they are quasi-stars[10] or similar objects, consisting of a black hole surrounded by a gaseous envelope.[11]
As active galactic nuclei
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LRDs were first selected by photometric methods because they are blue in ultraviolet and red in the optical spectrum.[5] 80% were found to have very broad Balmer emission lines, suggesting that they are active galactic nuclei (AGN) and host supermassive black holes at their center.[12] 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.[13][14] Scientists study the properties of AGNs to better understand supermassive black hole formation and how they contribute to the structure and dynamics of LRDs.[5] 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 accredited to the massive amounts of gas, dust, and electromagnetic energy that surrounds the AGN and supermassive black hole.[15] This region is also known as the accretion disk.
The gas in LRDs spins extremely fast.[2] Scientists argue that the gas is accelerated to these extreme speeds by spinning supermassive black holes.[2] A team using the Webb Telescope targeted LRDs in the 'Red Unknowns: Bright Infrared Extragalactic Survey',[16] observing rapid gas orbits of roughly 2,000,000 miles per hour (3,200,000 km/h) –a strong indicator of black hole accretion.[6]
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,[17] and little X-ray emission has been detected.[18][19] LRDs also show very weak time variability, while high variability is often seen in AGN observation.[20]
Observed properties
Several models have been proposed to explain the observed properties of LRDs.[21][22][23] The shape of the ultraviolet spectrum can be explained by the scattered AGN light[21][22] or by the gray dust extinction law.[23] A spectral energy distribution (SED) fit can be modeled for many LRDs from OB stellar clumps within dense dusty regions, with integrated light obscured at the 90-95% level.[24]
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.[6]
Most are extremely compact, averaging around 2% of the radius of the Milky Way.[4] A typical LRD has a radius no greater than 500 light-years, though many have radii smaller than 150 light-years.[25]
From a sample of 99 LRDs 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.[26] These suspected young black holes are among the smallest recorded, at 105 - 107 solar masses.[27]
Likely local analogues of LRDs were discovered in a sample of Green Pea galaxies (GP).[28] 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.[28]
RUBIES
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.[29][30][31] 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.[32] 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.[33]
Notable LRDs
The Cliff (RUBIES-UDS-154183)
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:[34]
the Balmer break, emission lines, and Hα absorption line are instead most plausibly explained by a black hole star (BH*) scenario, in which dense gas surrounds a powerful ionising source. In contrast to recently proposed BH* models of dust-reddened AGN, we show that spectral fits in the rest UV to near-infrared favour an intrinsically redder continuum over strong dust reddening. This may point to a super-Eddington accreting massive black hole or, possibly, the presence of (super)massive stars in a nuclear star cluster. The Cliff is the clearest evidence to date that at least some LRDs are not ultra-dense massive galaxies, and are instead powered by a central ionising source embedded in dense, absorbing gas.
CAPERS-LRD-z9
CAPERS-LRD-z9 is an LRD confirmed to be a broad-line active galactic nucleus (BLAGN) with the red-shift z = 9.288. It is the highest red-shift AGN known. CAPERS-LRD-z9 exhibits a prominent Balmer break and "provides strong evidence in support of the 'dense-gas-enshrouded AGN'" explanation.[35] These unusual Balmer jumps are notable properties of LDRs, causing difficulties in observations and spectroscopic analysis. Some LDRs exhibit a symmetrical density distribution of electron scattering, thus a gaseous envelope absorbing nebular spectra is a credible possibility. [27]
A2744–45924
The LRD A2744–45924 is located in the Abell 2744 field, and is the most optically-luminous LRD found by JWST.[36]
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."[30]
J1007_AGN
The LRD J1007_AGN has a red-shift z = 7.3, and is "embedded in an overdensity of eight nearby galaxies".[37]
Abell 2744-QSO1
Abell 2744-QSO1 is an LRD with z = 7.04. It was described as a "naked" black hole, because very few stars are in its vicinity.[1][38]
MoM-BH*-1
MoM-BH*-1 is a prominent LRB described as "a higher-redshift analogue of The Cliff".[34][39][9]
Further reading
- Boyle, Rebecca (March 2026). "Little Red Dots". Scientific American. Vol. 334, no. 3. New York: Springer Nature America. pp. 36–43.
- Ahart, Jenna (15 October 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.
- Lin, Xiaojing (2025). "The Discovery of Little Red Dots in the Local Universe: Signatures of Cool Gas Envelopes". arXiv:2507.10659v1 [astro-ph.GA].
- Inayoshi, Kohei; Maiolino, Roberto (20 February 2025). "Extremely Dense Gas around Little Red Dots and High-redshift Active Galactic Nuclei: A Nonstellar Origin of the Balmer Break and Absorption Features". The Astrophysical Journal Letters. 980 (2): L27. arXiv:2409.07805. Bibcode:2025ApJ...980L..27I. doi:10.3847/2041-8213/adaebd.
- Kokorev, Vasily; et al. (2023). "UNCOVER: A NIRSpec Identification of a Broad-line AGN at z = 8.50". The Astrophysical Journal Letters. 957 (1): L7. arXiv:2308.11610. Bibcode:2023ApJ...957L...7K. doi:10.3847/2041-8213/ad037a.
- Setton, David J.; et al. (2024). "Little Red Dots at an Inflection Point: Ubiquitous "V-Shaped" Turnover Consistently Occurs at the Balmer Limit". The Astrophysical Journal. 995 (1): 118. arXiv:2411.03424. Bibcode:2025ApJ...995..118S. doi:10.3847/1538-4357/ae1500.
- Xiao, Mengyuan; et al. (2025). "No [CII] or dust detection in two Little Red Dots at zspec > 7". Astronomy & Astrophysics. 700: A231. arXiv:2503.01945. doi:10.1051/0004-6361/202554361.
- Whalen, Kelly E.; Weaver, Kimberly A.; Hickox, Ryan C.; Lambrides, Erini (2026). "Limitations on Morphological Fitting for JWST "Little Red Dots"". The Astrophysical Journal. 998 (1): 133. arXiv:2509.21236. Bibcode:2026ApJ...998..133W. doi:10.3847/1538-4357/ae31e5.
- De Luca, Valerio; Del Grosso, Loris; Franciolini, Gabriele; Kritos, Konstantinos; Berti, Emanuele; d'Orazio, Daniel; Silk, Joseph (2025). "A cosmologist's take on Little Red Dots". arXiv:2512.19666 [astro-ph.CO].
