Do little red dots really form a distinct class of astronomical objects?

arXiv:2604.11677 · astro-ph.GA · Submitted 2026-04-13 · Read on arXiv

astro-ph.GA

Submitted: 2026-04-13

Updated: 2026-09-07

Comments: Accepted for publication in A&A

License: http://creativecommons.org/licenses/by-nc-sa/4.0/

The gist: JWST observations have identified a class of enigmatic sources known as little red dots (LRDs), interpreted as a distinct class of active galactic nuclei (AGNs) and host galaxies, whose black hole

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Abstract

JWST observations have identified a class of enigmatic sources known as little red dots (LRDs), interpreted as a distinct class of active galactic nuclei (AGNs) and host galaxies, whose black hole masses, AGN emissivities, stellar masses, and possible quasi-stars or black hole stars (BH*) suggest a previously unidentified class of extragalactic objects. However, two questions remain: is there a clear discontinuity between LRDs and field galaxies at the same epochs, and do LRDs form a homogeneous population? We address these issues with a continuous metric of the "LRDness" of galaxies, measuring their compactness (delta compact), the sharpness of the V-shaped spectral energy distribution (delta v-shape), and the strength of the broad Balmer emission. This approach, which avoids a binary "on-off" view, was applied to 48,000 (5,000) galaxies with photometric (spectroscopic) data over 750 arcmin squared. V-shape prominence correlates strongly with morphology, with no clear transition at the usual LRD threshold: the compact fraction rises with V-shape intensity. Similarly, broad H-alpha strength increases with V-shape sharpness and compactness. The [N II] deficit is not exclusive to LRDs, but a global property of compact, metal-poor galaxies. Only a minority of LRDs (the 3% most extreme) show a prominent Balmer break (greater than 3) of potentially non-stellar origin. LRDs and non-LRDs follow a similar Balmer decrement versus V-shape trend, suggesting a common origin consistent with dust attenuation, reinforced by the agreement between observed Balmer ratios and attenuated Case B predictions. The inferred dust mass (4-7 x 10 4 M sun) is low enough to explain ALMA non-detections. We conclude that most LRDs are not a separate class, but rather the extreme tail of a continuous distribution of galaxies and broad H-alpha emitters, consistent with a classical broad-line region and dust component.

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