Behind the dust veil: A panchromatic view of an optically dark galaxy at z = 4.82
Koekemoer, Anton M.; Daddi, Emanuele; Schinnerer, Eva; Gillman, Steven; Faisst, Andreas; Magdis, Georgios E.; Chen, Chian-Chou; Finoguenov, Alexis; Jin, Shuowen; Wang, Wei-Hao; Weaver, John R.; Kokorev, Vasily; Sillassen, Nikolaj B.; Brinch, Malte; Shuntov, Marko; Gobat, Raphael; Valentino, Francesco; Hodge, Jacqueline; Enia, Andrea; Gozaliasl, Ghassem; Talia, Margherita; Andika, Irham T.; Magnelli, Benjamin; Rizzo, Francesca; Cochrane, Rachel; Rich, Michael; Gao, Zhen-Kai; Algera, Hiddo; Knudsen, Kirsten; Blánquez-Sesé, David; Gomez-Guijarro, Carlos; Hayward, Chris; Merchant, Maya
Denmark, Netherlands, Chile, France, Sweden, Finland, Germany, Taiwan, United States, Japan, United Kingdom, Italy
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 Tdust = 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.