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Arxiv: An optically-dark merging system at z~6 detected by JWST Published: 5/7/2024 9:00:01 PM Updated: 5/7/2024 9:00:01 PM

Paper abstract: Near- to mid-Infrared observations (from Spitzer and JWST) have revealed ahidden population of galaxies at redshift z=3-6, called optically-dark objects,which are believed to be massive and dusty star-formers. While optically-darksources are widely recognized as a significant component of the stellar massfunction, the history of their stellar mass assembly remains unexplored.However, they are thought to be the progenitors of the more massive early-typegalaxies found in present-day groups and clusters. It is thus important toexamine the possible connection between dark sources and merging events, inorder to understand the environment in which they live. Here, we report oursearch for close companions in a sample of 19 optically-dark objects identifiedin the SMACS0723 JWST deep field. They were selected in the NIRCam F444W bandand undetected below 2mu. We restrict our analysis to the reddest (i.e.F277W-F444W> 1.3) and brightest (F444W< 26 mag) objects. We have identified anoptically-dark source showing a very close companion (<0.5"). The spatiallyresolved SED fitting procedure indicates that all components lying within 1.5"from the dark source are indeed at z~5.7. Tidal features (leading to a whaleshaped morphology) corroborate the hypothesis that the dark source is the mostmassive (log(M/Msun)>10.3) and dusty (Av~3 at the core) system of an ongoingmerger with a mass ratio of ~10. Similar merging systems are identified in theSERRA simulations, allowing us to reconstruct their stellar mass assemblyhistory and predict their molecular gas properties The discovery of mergerswithin dark galaxies at the end of the Epoch of Reionization underscores theimportance of conducting a statistical search for additional candidates in deepNIRCam fields. Such research will aid in understanding the role of mergingprocesses during the obscured phase of stellar mass accumulation.