Active galactic nuclei-heated dust revealed in "little red dots"
I. Delvecchio, E. Daddi, B. Magnelli, D. Elbaz, M. Giavalisco, A. Traina, G. Lanzuisi, H. B. Akins, S. Belli, C. M. Casey, F. Gentile, C. Gruppioni, F. Pozzi, G. Zamorani
TL;DR
This paper uses a large, uniformly selected sample of 302 little red dots (LRDs) from JWST fields to perform a median stacking analysis with NIRCam, MIRI, and ALMA data. By fitting the median stacked spectral energy distribution with and without an AGN component, the authors find clear evidence for hot-dust emission consistent with AGN-heated dust, typically around $T\approx820$ K, and they infer that many LRDs host Compton-thick gas within the dust sublimation radius, explaining their X-ray faintness. The results suggest that more than half of LRDs contain AGN-heated dust, regardless of whether the optical/UV continuum is stellar- or AGN-dominated, while the ALMA upper limits constrain the cold dust content and support a scenario in which X-ray obscuration occurs in dust-free gas inside $R_{ m sub}$. The study highlights the heterogeneity of LRDs, reconciles their multiwavelength properties, and provides empirical benchmarks for AGN–galaxy co-evolution at $z\sim4$–$8$, though a full radiative-transfer treatment and deeper individual observations are needed to fully unravel formation channels of hot dust in these systems.
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_{\rm rest}$$\sim$ 3$μ$m, which is best explained by a standard dusty active galactic nucleus (AGN) structure. Although LRDs are likely to be a heterogeneous population, our findings suggest that most ($\gtrsim$50 %) LRDs show AGN-heated dust emission, regardless of whether the optical and ultraviolet (UV) continua are stellar or AGN-dominated. In either case, the best-fit dusty-AGN SED, combined with the lack of X-ray detection in the deep Chandra stacks, suggests that Compton-thick ($N_{\rm H}$$>$3$\times$10$^{24}$ cm$^{-2}$) gas obscuration is common, and likely confined within the dust sublimation radius ($R$$_{\rm sub}$$\sim$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.