Active galactic nuclei-heated dust revealed in “little red dots”

¹ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, I-40129 Bologna, Italy
² Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, 91191 Gif-sur-Yvette, France
³ Astronomy Department, University of Massachusetts, Amherst, MA 01003, USA
⁴ The University of Texas at Austin, 2515 Speedway Boulevard Stop C1400, Austin, TX 78712, USA
⁵ University of Bologna, Department of Physics and Astronomy (DIFA), Via Gobetti 93/2, I-40129 Bologna, Italy
⁶ Department of Physics, University of California, Santa Barbara, Santa Barbara, CA 93106, USA

^★ Corresponding author: ivan.delvecchio@inaf.it

Received: 10 September 2025
Accepted: 27 October 2025

Abstract

Little red dots (LRDs) are a puzzling population of extragalactic sources whose origin is highly debated. In this work, we performed a comprehensive stacking analysis of NIRCam, MIRI, and ALMA images of a large and homogeneously selected sample of LRDs from multiple JWST Legacy fields. We report clear evidence of hot-dust emission in the median stacked spectral energy distribution (SED) that features a rising near-infrared continuum up to rest-frame λ_rest ∼ 3 μm, which is best explained by a standard dusty active galactic nucleus (AGN) structure. Although LRDs are likely to be a heterogeneous population, our findings suggest that most (≳50%) LRDs show AGN-heated dust emission, regardless of whether the optical and ultraviolet (UV) continua are stellar or AGN-dominated. In either case, the best-fit dusty-AGN SED, combined with the lack of X-ray detection in the deep Chandra stacks, suggests that Compton-thick (N_H > 3 × 10²⁴ cm⁻²) gas obscuration is common, and likely confined within the dust sublimation radius (R_sub ∼ 0.1 pc). Therefore, we argue that AGN-heated dust does not directly obscure either the optical-UV continuum or the broad-line region emission, in order to explain the observed blue UV slopes and prominent Balmer features. While a gas-dust displacement is in line with several models, the formation scenario (in-situ or ex-situ) of this pre-enriched hot dust remains unclear.