Do little red dots really form a distinct class of astronomical objects?
6 affiliations
- 1Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM
- 2INAF – Osservatorio di Astrofisica e Scienza dello Spazio
- 3University of Massachusetts Amherst
- 4Department of Physics and Astronomy, Colby College
- 5Key Laboratory of Modern Astronomy and Astrophysics, Nanjing University, Ministry of Education
- 6School of Astronomy and Space Science, Nanjing University
Abstract
Observations with the James Webb Space Telescope have identified a previously unidentified class of enigmatic sources, known as little red dots (LRDs), which are relatively abundant in space. These sources have been interpreted as a distinct class of active galactic nuclei (AGNs) and host galaxies whose structural properties do indeed suggest the existence of a previously unidentified class of extragalactic objects. Such properties include the mass of the supermassive black hole and the emissivity from the associated AGN, as well as the stellar mass of the host galaxy or the possible presence of so-called quasi-stars or black hole stars (BH ∗ ). However, there are two issues that have yet to be addressed before invoking a new paradigm: 1) whether there a clear discontinuity between the properties of the LRDs and field galaxies at the same cosmic epochs; and 2) whether LRDs form a single homogeneous population. In this work, we address these two issues by introducing a continuous metric to evaluate the “LRDness” of galaxies. We measured their compactness ( δ compact ), the sharpness of the V-shaped ( δ v-shape ) spectral energy distribution (SED), and the strength of the broad Balmer line emission. We applied this approach, which avoids imposing a binary “on–off” view of the population, to a sample of ≈48 000 (≈5000) galaxies with photometric (spectroscopic) information, selected over an area of ≈750 arcmin 2 . We find that the prominence of the V-shaped SED exhibits a strong correlation with morphology without any clear transition at the commonly used LRD selection threshold: the fraction of compact galaxies rises in line with the V-shape intensity. Similarly, the strength of the H α broad line component increases with the V-shape sharpness and with compactness. We also note that the deficit of [N II ] itself is not an exclusive feature of LRDs, but a global property of compact, metal-poor galaxies. Finally, only a minority of LRDs (i.e., the 3% most extreme ones) present a prominent Balmer break of potentially non-stellar origin (greater than 3). Overall, LRDs and non-LRDs show a similar Balmer decrement versus V-shape evolution, suggesting a common origin consistent with a dust attenuation origin. The inferred dust mass (≈4–7 × 10 4 M ⊙ ) is low enough to explain ALMA non-detections, even if they are fully originating from stellar attenuation. The dust-driven origin is reinforced by the agreement between the observed Balmer ratios and attenuated Case B predictions. In conclusion, our results suggest that (apart from a minority, i.e., a few percent) LRDs do not generally represent a separate class of objects; rather, they are likely to make up the extreme tail of a continuous distribution of galaxies and broad H α emitters. Finally, we find that most LRDs appear to be consistent with a classical broad-line region and a dust component.
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BibTeX
@article{billand2026little,
title = {{Do little red dots really form a distinct class of astronomical objects?}},
author = {Jean-Baptiste Billand and D. Elbaz and Maximilien Franco and Fabrizio Gentile and Emanuele Daddi and Mauro Giavalisco and Dale D. Kocevski and Joseph S. W. Lewis and B. Magnelli and Valentina Sangalli and Maxime Tarrasse},
journal = {Astronomy and Astrophysics},
year = {2026},
month = sep,
doi = {10.1051/0004-6361/202660400},
url = {https://doi.org/10.1051/0004-6361/202660400},
note = {CC BY},
}
RIS
TY - JOUR
TI - Do little red dots really form a distinct class of astronomical objects?
AU - Jean-Baptiste Billand
AU - D. Elbaz
AU - Maximilien Franco
AU - Fabrizio Gentile
AU - Emanuele Daddi
AU - Mauro Giavalisco
AU - Dale D. Kocevski
AU - Joseph S. W. Lewis
AU - B. Magnelli
AU - Valentina Sangalli
AU - Maxime Tarrasse
PY - 2026
DA - 2026/09/07
JO - Astronomy and Astrophysics
DO - 10.1051/0004-6361/202660400
UR - https://doi.org/10.1051/0004-6361/202660400
AB - Observations with the James Webb Space Telescope have identified a previously unidentified class of enigmatic sources, known as little red dots (LRDs), which are relatively abundant in space. These sources have been interpreted as a distinct class of active galactic nuclei (AGNs) and host galaxies whose structural properties do indeed suggest the existence of a previously unidentified class of extragalactic objects. Such properties include the mass of the supermassive black hole and the emissivity from the associated AGN, as well as the stellar mass of the host galaxy or the possible presence of so-called quasi-stars or black hole stars (BH ∗ ). However, there are two issues that have yet to be addressed before invoking a new paradigm: 1) whether there a clear discontinuity between the properties of the LRDs and field galaxies at the same cosmic epochs; and 2) whether LRDs form a single homogeneous population. In this work, we address these two issues by introducing a continuous metric to evaluate the “LRDness” of galaxies. We measured their compactness ( δ compact ), the sharpness of the V-shaped ( δ v-shape ) spectral energy distribution (SED), and the strength of the broad Balmer line emission. We applied this approach, which avoids imposing a binary “on–off” view of the population, to a sample of ≈48 000 (≈5000) galaxies with photometric (spectroscopic) information, selected over an area of ≈750 arcmin 2 . We find that the prominence of the V-shaped SED exhibits a strong correlation with morphology without any clear transition at the commonly used LRD selection threshold: the fraction of compact galaxies rises in line with the V-shape intensity. Similarly, the strength of the H α broad line component increases with the V-shape sharpness and with compactness. We also note that the deficit of [N II ] itself is not an exclusive feature of LRDs, but a global property of compact, metal-poor galaxies. Finally, only a minority of LRDs (i.e., the 3% most extreme ones) present a prominent Balmer break of potentially non-stellar origin (greater than 3). Overall, LRDs and non-LRDs show a similar Balmer decrement versus V-shape evolution, suggesting a common origin consistent with a dust attenuation origin. The inferred dust mass (≈4–7 × 10 4 M ⊙ ) is low enough to explain ALMA non-detections, even if they are fully originating from stellar attenuation. The dust-driven origin is reinforced by the agreement between the observed Balmer ratios and attenuated Case B predictions. In conclusion, our results suggest that (apart from a minority, i.e., a few percent) LRDs do not generally represent a separate class of objects; rather, they are likely to make up the extreme tail of a continuous distribution of galaxies and broad H α emitters. Finally, we find that most LRDs appear to be consistent with a classical broad-line region and a dust component.
SN - 0004-6361
SN - 1432-0746
ER -