James Webb Space Telescope Feed Post

An optically-dark merging system at z~6 detected by JWST

Left panel: RGB image of the KLAMA system (Red=F444W, Green=F277W, Blue=F200W). For reference, the yellow square marks the field of view of NIRSpec IFU (3 "×3 "). We indicate the three components of the galaxy group with capital letters (A, B, and C). KLAMA is the reddest source (A). Right panel: F356W highly contrasted image of the same region shown in the left panel. The three components are indicated. Interestingly, a diffuse emission clearly rises as a bridge (marked with the red arrow) connecting the main system (A+B) to the tail (C). The green circles represent the apertures (equal to the FWHM of the F444W PSF) and the positions used to extract the photometry for the Bagpipes fit. Abstract: Near- to mid-Infrared observations (from Spitzer and JWST) have revealed a hidden population of galaxies at redshift z=3-6, called optically-dark objects, which are believed to be massive and dusty star-formers. While optically-dark sources are widely recognized as a significant component of the stellar mass function, the history of their stellar mass assembly remains unexplored. However, they are thought to be the progenitors of the more massive early-type galaxies found in present-day groups and clusters. It is thus important to examine the possible connection between dark sources and merging events, in order to understand the environment in which they live. Here, we report our search for close companions in a sample of 19 optically-dark objects identified in the SMACS0723 JWST deep field. They were selected in the NIRCam F444W band and 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 an optically-dark source showing a very close companion (<0.5"). The spatially resolved 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 whale shaped morphology) corroborate the hypothesis that the dark source is the most massive (log(M/Msun)>10.3) and dusty (Av~3 at the core) system of an ongoing merger with a mass ratio of ~10. Similar merging systems are identified in the SERRA simulations, allowing us to reconstruct their stellar mass assembly history and predict their molecular gas properties The discovery of mergers within dark galaxies at the end of the Epoch of Reionization underscores the importance of conducting a statistical search for additional candidates in deep NIRCam fields. Such research will aid in understanding the role of merging processes during the obscured phase of stellar mass accumulation.