Behind the dust veil: A panchromatic view of an optically dark galaxy at z = 4.82

¹ Cosmic Dawn Center (DAWN), Copenhagen, Denmark
² DTU-Space, Technical University of Denmark, Elektrovej 327, DK-2800 Kgs. Lyngby, Denmark
³ Niels Bohr Institute, University of Copenhagen, Jagtvej 128, DK-2200 Copenhagen, Denmark
⁴ Leiden Observatory, Leiden University, NL-2300 RA Leiden, The Netherlands
⁵ Instituto de Física, Pontificia Universidad Católica de Valparaíso, Casilla 4059, Valparaíso, Chile
⁶ Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, Paris, France
⁷ Department of Space, Earth, & Environment, Chalmers University of Technology, Chalmersplatsen 4, 412 96 Gothenburg, Sweden
⁸ Department of Physics, University of Helsinki, PO Box 64 00014 Helsinki, Finland
⁹ Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany
¹⁰ Academia Sinica Institute of Astronomy and Astrophysics (ASIAA), No. 1, Section 4, Roosevelt Rd., Taipei 106216, Taiwan
¹¹ Department of Astronomy, University of Massachusetts, Amherst, MA 01003, USA
¹² Hiroshima Astrophysical Science Center, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan
¹³ National Astronomical Observatory of Japan, 2-21-1, Osawa, Mitaka, Tokyo, Japan
¹⁴ Technical University of Munich, TUM School of Natural Sciences, Department of Physics, James-Franck-Str. 1, D-85748 Garching, Germany
¹⁵ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, D85748 Garching, Germany
¹⁶ Instituto de Física y Astronomía, Universidad de Valparaíso, Avda. Gran Bretana 1111, Valparaíso, Chile
¹⁷ Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
¹⁸ Department of Astronomy, Columbia University, New York, NY 10027, USA
¹⁹ University of Bologna– Department of Physics and Astronomy “Augusto Righi” (DIFA), Via Gobetti 93/2, I-40129 Bologna, Italy
²⁰ INAF–Osservatorio di Astrofisica e Scienza dello Spazio, Via Gobetti 93/3, I-40129 Bologna, Italy
²¹ Caltech/IPAC, 1200 E. California Boulevard, Pasadena, CA 91125, USA
²² Department of Computer Science, Aalto University, PO Box 15400 Espoo 00 076, Finland
²³ Center for Computational Astrophysics, Flatiron Institute, 162 Fifth Avenue, New York, NY 10010, USA
²⁴ Department of Astronomy, The University of Texas at Austin, Austin, TX 78712, USA
²⁵ Kapteyn Astronomical Institute, University of Groningen, Landleven 12, 9747 AD Groningen, The Netherlands
²⁶ University of Bologna, Department of Physics and Astronomy (DIFA), Via Gobetti 93/2, I-40129 Bologna, Italy
²⁷ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
²⁸ Department of Physics & Astronomy, University of California Los Angeles, 430 Portola Plaza, Los Angeles, CA 90095, USA

^⋆ Corresponding authors; nbsi@space.dtu.dk, shuji@dtu.dk

Received: 8 November 2024
Accepted: 12 December 2024

Abstract

Optically dark dusty star-forming galaxies (DSFGs) play an essential role in massive galaxy formation at early cosmic time; however, their nature remains elusive. Here, we present a detailed case study of all the baryonic components of a z = 4.821 DSFG, XS55. Selected from the ultra-deep COSMOS-XS 3 GHz map with a red SCUBA-2 450 μm/850 μm colour, XS55 was followed up with ALMA 3 mm line scans and spectroscopically confirmed to be at z = 4.821 via detections of the CO(5-4) and [CI](1-0) lines. JWST/NIRCam imaging reveals that XS55 is a F150W drop-out with a red F277W/F444W colour and a complex morphology: a compact central component embedded in an extended structure with a likely companion. XS55 is tentatively detected in X-rays with both Chandra and XMM-Newton, suggesting an active galactic nucleus nature. By fitting a panchromatic spectral energy distribution spanning from near-infrared to radio wavelengths, we reveal that XS55 is a massive main-sequence galaxy with a stellar mass of M_* = (5 ± 1)×10¹⁰ M_⊙ and a star formation rate of SFR = 540 ± 177 M_⊙ yr⁻¹. The dust of XS55 is optically thick in the far-infrared with a surprisingly cold dust temperature of T_dust = 33 ± 2 K, making XS55 one of the coldest DSFGs at z > 4 known to date. This work unveils the nature of a radio-selected F150W drop-out, suggesting the existence of a population of DSFGs hosting active black holes embedded in optically thick dust.