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

TL;DR

This study challenges the idea that Little Red Dots are a distinct class of astronomical objects, showing they are part of a continuous galaxy population with similar physical properties.

Contribution

Introduces a continuous metric to evaluate LRDs, demonstrating they are the extreme end of a galaxy distribution rather than a separate class.

Findings

V-shape prominence correlates with galaxy morphology.

Broad Hα strength increases with V-shape sharpness and compactness.

Most LRDs are part of a continuous galaxy distribution, not a separate class.

Abstract

JWST observations have identified a class of enigmatic sources known as "Little Red Dots" (LRDs). These have been interpreted as a distinct class of active galactic nuclei (AGN) and host galaxies, potentially involving "quasi-stars" or Black Hole stars (BH*). However, two questions remain: is there a clear discontinuity between LRDs and field galaxies, and do LRDs form a homogeneous population? In this work, we address these issues by introducing a continuous metric to evaluate the "LRDness" of galaxies. We measure their compactness ($\delta_{compact}$), the sharpness of the V-shaped spectral energy distribution ($\delta_{v-shape}$), and the strength of the broad Balmer line emission. We apply this approach to a sample of ~48,000 galaxies with photometric and ~5,000 with spectroscopic information, selected over ~750 arcmin^2. We find that V-shape prominence correlates strongly with morphology without a clear transition at common LRD selection thresholds: the fraction of compact galaxies rises continuously with V-shape intensity. Similarly, broad H$\alpha$ strength increases with both V-shape sharpness and compactness. The [N II] deficit is not an exclusive feature of LRDs but a global property of compact, metal-poor galaxies. Only the 3% most extreme LRDs present a prominent Balmer break (>3) of potentially non-stellar origin. LRDs and non-LRDs follow similar trends in the evolution of the Balmer decrement with V-shape sharpness, suggesting a shared physical origin, likely dust attenuation. Estimated dust masses (~4-7 x 10^4 M_{sun}) and luminosities are low enough to account for their non-detection by ALMA. We conclude that most LRDs do not represent a separate class of objects, 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 attenuation.