A Confirmed Deficit of Hot and Cold Dust Emission in the Most Luminous Little Red Dots

Abstract Luminous broad H α emission and red rest-optical spectral energy distributions (SEDs) are the hallmark of compact little red dots (LRDs), implying highly attenuated dusty starbursts and/or obscured active galactic nuclei (AGN). However, the lack of observed far-infrared (FIR) emission has proved difficult to reconcile with the implied attenuated luminosity in these models. Here, we utilize deep new Atacama Large Millimeter/submillimeter Array 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 rest = 1–4 μ m imply flat slopes in the rest-IR, ruling out a contribution from hot ( T ≳ 500 K) dust. Similarly, FIR nondetections rule out any appreciable cold ( T ≲ 75 K) dust component. Assuming energy balance, these observations are inconsistent with the typical FIR dust emission of dusty starbursts and quasar tori, which usually show a mixture of cold and hot dust. Additionally, our [C ii ] nondetections rule out typical dusty starbursts. We compute empirical maximum IR SEDs and find that both LRDs must have <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:msub> <mml:mrow> <mml:mi>L</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">IR</mml:mi> </mml:mrow> </mml:msub> <mml:mo>/</mml:mo> <mml:msub> <mml:mrow> <mml:mi>L</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>⊙</mml:mo> </mml:mrow> </mml:msub> <mml:mo stretchy="false">)</mml:mo> <mml:mo>≲</mml:mo> <mml:mn>12.2</mml:mn> </mml:math> at the 3 σ 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.

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Publication Details

Journal
The Astrophysical Journal Letters
Published
2025-09-12
DOI
https://doi.org/10.3847/2041-8213/ade78b
Citations
29
Primary Topic
Galaxies: Formation, Evolution, Phenomena
Type
article
Field-Weighted Citation Impact
33.49

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article

A Confirmed Deficit of Hot and Cold Dust Emission in the Most Luminous Little Red Dots

Ian McConachie, Pratika Dayal, Seiji Fujimoto, Jorryt Matthee et al.
29 citations
The Astrophysical Journal Letters
Galaxies: Formation, Evolution, Phenomena
33.49
article

A Confirmed Deficit of Hot and Cold Dust Emission in the Most Luminous Little Red Dots

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

Abstract

Abstract Luminous broad H α emission and red rest-optical spectral energy distributions (SEDs) are the hallmark of compact little red dots (LRDs), implying highly attenuated dusty starbursts and/or obscured active galactic nuclei (AGN). However, the lack of observed far-infrared (FIR) emission has proved difficult to reconcile with the implied attenuated luminosity in these models. Here, we utilize deep new Atacama Large Millimeter/submillimeter Array 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 rest = 1–4 μ m imply flat slopes in the rest-IR, ruling out a contribution from hot ( T ≳ 500 K) dust. Similarly, FIR nondetections rule out any appreciable cold ( T ≲ 75 K) dust component. Assuming energy balance, these observations are inconsistent with the typical FIR dust emission of dusty starbursts and quasar tori, which usually show a mixture of cold and hot dust. Additionally, our [C ii ] nondetections rule out typical dusty starbursts. We compute empirical maximum IR SEDs and find that both LRDs must have <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:msub> <mml:mrow> <mml:mi>L</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">IR</mml:mi> </mml:mrow> </mml:msub> <mml:mo>/</mml:mo> <mml:msub> <mml:mrow> <mml:mi>L</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>⊙</mml:mo> </mml:mrow> </mml:msub> <mml:mo stretchy="false">)</mml:mo> <mml:mo>≲</mml:mo> <mml:mn>12.2</mml:mn> </mml:math> at the 3 σ 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.

The Astrophysical Journal LettersVol. 991(1)
National Science Foundation, National Aeronautics and Space Administration, Brinson Foundation, Space Telescope Science Institute, National Radio Astronomy Observatory, National Research Foundation, Schweizerischer Nationalfonds zur Förderung der Wissenschaftlichen Forschung, Danmarks Grundforskningsfond, United States-Israel Binational Science Foundation, Nuclear Safety and Security Commission, Korea Astronomy and Space Science Institute, National Astronomical Observatory of Japan, Goddard Space Flight Center, National Institutes of Natural Sciences, Staatssekretariat für Bildung, Forschung und Innovation
Openalex Percentile: Top 0%
Galaxies: Formation, Evolution, Phenomena
33.49
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