2023MNRAS.518L..19R

A wealth of extragalactic populations completely missed at UV-optical wavelengths has been identified in the last decade, combining the deepest HST and Spitzer observations. These dark sources are thought to be dusty and star-forming systems at 3 < z < 5, and major contributors to the stellar mass build up. In this letter, we report an investigation of the deep JWST survey in the SMACS0723 cluster, analysing NIRCam and MIRI images. We search for sources in the F444W band that are undetected in the F200W catalogues. We characterize the properties of these sources via detailed Spectral Energy Distribution (SED) modelling, accounting for a wide set of parameters and star formation histories, after a careful determination of their photometry. Among a robust sample of 20 candidates, we identify a mixed population of very red sources. We highlight the identification of evolved systems, with stellar masses M_* ∼ 10^9–11 M_⊙ at 8 < z < 13 characterized by unexpectedly important dust content at those epochs (A_V up to ∼5.8 mag), challenging current model predictions. We further identify an extremely red source (F200W-F440W ∼ 7 mag) that can be reproduced only by the spectrum of a passive, quenched galaxy of M_* ∼ 10^11.56 M_⊙ at z ∼ 5, filled of dust (A_V ∼ 5 mag).