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

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 3GHz map with a red SCUBA-2 450\(\mu\)m/850\(\mu\)m colour, XS55 was followed up with ALMA 3mm 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-dropout with 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 (AGN) nature. By fitting a panchromatic SED spanning NIR to radio wavelengths, we revealed that XS55 is a massive main-sequence galaxy with a stellar mass of \(M_\ast=(5\pm1)\times10^{10}\,{\rm M_\odot}\) and a star formation rate of \({\rm SFR}=540\pm177~{\rm M_\odot\,yr^{-1}}\). The dust of XS55 is optically thick in the far infrared (FIR) with a surprisingly cold dust temperature of \(T_{\rm dust}=33\pm2\,{\rm K}\), making XS55 one of the coldest DSFGs at \(z>4\) known to date. This work unveils the nature of a radio-selected F150W-dropout, suggesting the existence of a population of DSFGs hosting active black holes embedded in optically thick dust.