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A confirmed deficit of hot and cold dust emission in the most luminous Little Red Dots

David J. Setton, Jenny E. Greene, Justin S. Spilker, Christina C. Williams, Ivo Labbe, Yilun Ma, Bingjie Wang, Katherine E. Whitaker, Joel Leja, Anna de Graaff, Stacey Alberts, Rachel Bezanson, Leindert A. Boogaard, Gabriel Brammer, Sam E. Cutler, Nikko J. Cleri, Olivia R. Cooper, Pratika Dayal, Seiji Fujimoto, Lukas J. Furtak, Andy D. Goulding, Michaela Hirschmann, Vasily Kokorev, Michael V. Maseda, Ian McConachie, Jorryt Matthee, Tim B. Miller, Rohan P. Naidu, Pascal A. Oesch, Richard Pan, Sedona H. Price, Katherine A. Suess, John R. Weaver, Mengyuan Xiao, Yunchong Zhang, Adi Zitrin

TL;DR

This study tests whether the most luminous Little Red Dots (LRDs) can be explained by heavily reddened starbursts or AGN by placing stringent infrared luminosity constraints on two bright LRDs using deep ALMA, JWST/MIRI, and archival data. By comparing rest-frame optical SED-based energy-balance predictions to IR limits, the authors show no evidence for significant hot or cold dust emission, with log(L_IR/L⊙) ≤ 12.2 (3σ) for both targets. The results contradict the high IR outputs implied by spectroscopic rest-optical fits and argue against intrinsically blue, heavily reddened engines; instead, an intrinsically redder LRD SED is favored. These findings imply a revised view of LRD energy sources and dust content, impacting models of early galaxy and AGN evolution.

Abstract

Luminous broad H$α$ emission and red rest-optical SEDs are the hallmark of compact Little Red Dots (LRDs), implying highly attenuated dusty starbursts and/or obscured active galactic nuclei. However, the lack of observed FIR emission has proved difficult to reconcile with the implied attenuated luminosity in these models. Here, we utilize deep new ALMA imaging, new and existing JWST/MIRI imaging, and archival Spitzer/Herschel imaging of two of the rest-optically brightest LRDs ($z=3.1$ and $z=4.47$) to place the strongest constraints on the IR luminosity in LRDs to date. The detections at $λ_\mathrm{rest}=1-4 \ μ$m imply flat slopes in the rest-IR, ruling out a contribution from hot ($T\gtrsim500$ K) dust. Similarly, FIR non-detections rule out any appreciable cold ($T\lesssim75$ K) dust component. Assuming energy balance, these observations are inconsistent with the typical FIR dust emission of dusty starbursts and quasar torii, which usually show a mixture of cold and hot dust. Additionally, our [$\mathrm{C}_{II}$] non-detections rule out typical dusty starbursts. We compute empirical maximum IR SEDs and find that both LRDs must have $\log(L_\mathrm{IR}/L_\odot) \lesssim 12.2$ at the $3σ$ level. These limits are in tension with the predictions of rest-optical spectrophotometric fits, be they galaxy only, AGN only, or composite. It is unlikely that LRDs are highly dust-reddened intrinsically blue sources with a dust temperature distribution that conspires to avoid current observing facilities. Rather, we favor an intrinsically redder LRD SED model that alleviates the need for strong dust attenuation.

A confirmed deficit of hot and cold dust emission in the most luminous Little Red Dots

TL;DR

This study tests whether the most luminous Little Red Dots (LRDs) can be explained by heavily reddened starbursts or AGN by placing stringent infrared luminosity constraints on two bright LRDs using deep ALMA, JWST/MIRI, and archival data. By comparing rest-frame optical SED-based energy-balance predictions to IR limits, the authors show no evidence for significant hot or cold dust emission, with log(L_IR/L⊙) ≤ 12.2 (3σ) for both targets. The results contradict the high IR outputs implied by spectroscopic rest-optical fits and argue against intrinsically blue, heavily reddened engines; instead, an intrinsically redder LRD SED is favored. These findings imply a revised view of LRD energy sources and dust content, impacting models of early galaxy and AGN evolution.

Abstract

Luminous broad H emission and red rest-optical SEDs are the hallmark of compact Little Red Dots (LRDs), implying highly attenuated dusty starbursts and/or obscured active galactic nuclei. However, the lack of observed FIR emission has proved difficult to reconcile with the implied attenuated luminosity in these models. Here, we utilize deep new ALMA imaging, new and existing JWST/MIRI imaging, and archival Spitzer/Herschel imaging of two of the rest-optically brightest LRDs ( and ) to place the strongest constraints on the IR luminosity in LRDs to date. The detections at m imply flat slopes in the rest-IR, ruling out a contribution from hot ( K) dust. Similarly, FIR non-detections rule out any appreciable cold ( K) dust component. Assuming energy balance, these observations are inconsistent with the typical FIR dust emission of dusty starbursts and quasar torii, which usually show a mixture of cold and hot dust. Additionally, our [] non-detections rule out typical dusty starbursts. We compute empirical maximum IR SEDs and find that both LRDs must have at the level. These limits are in tension with the predictions of rest-optical spectrophotometric fits, be they galaxy only, AGN only, or composite. It is unlikely that LRDs are highly dust-reddened intrinsically blue sources with a dust temperature distribution that conspires to avoid current observing facilities. Rather, we favor an intrinsically redder LRD SED model that alleviates the need for strong dust attenuation.

Paper Structure

This paper contains 5 sections, 1 figure.

Table of Contents

  1. Introduction
  2. Data
  3. The two most H$\alpha$ luminous LRDs in the southern sky
  4. Restframe-optical SED modeling
  5. IR Constraints

Figures (1)

  • Figure 1: A demonstration of the fundamental assumption that goes into the vast majority of LRD models, using the composite model from Labbe2024_monster for A2744-45924. In blue, we show the intrinsic, unattenuated galaxy+AGN model, which, via a combination of reddening and scattering, is observed in the rest optical as the NIRSpec/PRISM spectrum (orange). The total attenuated luminosity for both the galaxy and AGN components are listed in the bottom right. Assuming energy balance, this model, and models like it, predict significant FIR output where the dust that is being heated by the intrinsically blue engine of the LRD, regardless of whether that engine is primarily driven by a starburst or an AGN.