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

First listed 2025-03-03 | Last updated 2025-03-03

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.

Short digest

Deep MIRI imaging plus new ALMA continuum and [C II] spectroscopy on two luminous LRDs at z=3.1 (A2744-45924) and z=4.47 (RUBIES-BLAGN-1) set the tightest IR constraints yet. Rest-frame 1–4 μm detections show a flat IR slope, excluding hot (T≳500 K) dust, while far-IR non-detections exclude a cold (T≲75 K) component and typical [C II]-bright dusty starbursts. Empirical maximal SEDs require log(L_IR/L_sun) ≲ 12.2 (3σ), in strong tension with energy-balance predictions from reddened galaxy, AGN, or composite fits. The authors favor intrinsically red LRD SEDs over heavily dust-reddened blue engines, reshaping how these compact sources are interpreted.

Key figures to inspect

• Figure 2: Inspect the side-by-side cutouts to see mid-IR detections but absent ALMA continuum and [C II] for both A2744-45924 and RUBIES-BLAGN-1—this visually anchors the dust and line non-detections driving the conclusions.
• Figure 3: Compare IR SED data/limits against galaxy-only and composite (SKIRTOR torus + Draine cold dust) predictions; note how both hot and cold components overshoot the new MIRI/ALMA constraints.
• Figure 4: Trace the allowed contribution of single-temperature modified blackbodies (20–800 K); observe how rest-frame ≈1–4 μm rules out hot dust and ALMA/Herschel limits cap any warm/cold components, with the Akins et al. stacked LRD SED shown for context.
• Figure 1: Review the energy-balance premise using A2744-45924—models that redden an intrinsically blue engine predict substantial FIR re-emission that is incompatible with the new limits.