Early Results from GLASS-JWST. XX. Unveiling a Population of “Red Excess” Galaxies in Abell2744 and in the Coeval Field

We combine JWST/NIRCam imaging and MUSE data to characterize the properties of galaxies in different environmental conditions in the cluster Abell2744 ( z = 0.3064) and in its immediate surroundings. We investigate how galaxy colors, morphology, and star-forming fractions depend on wavelength and on different parameterizations of environment. Our most striking result is the discovery of a “red excess” population in F200W−F444W colors in both the cluster regions and the field. These galaxies have normal F115W−F150W colors but are up to 0.8 mag redder than red sequence galaxies in F200W−F444W. They also have rather blue rest-frame B − V colors. Galaxies in the field and at the cluster virial radius are overall characterized by redder colors, but galaxies with the largest color deviations are found in the field and in the cluster core. Several results suggest that mechanisms taking place in these regions might be more effective in producing these colors. Looking at their morphology, many cluster galaxies show signatures consistent with ram pressure stripping, while field galaxies have features resembling interactions and mergers. Our hypothesis is that these galaxies are characterized by dust-enshrouded star formation: a JWST/NIRSpec spectrum for one of the galaxies is dominated by a strong PAH at 3.3 μ m, suggestive of dust-obscured star formation. Larger spectroscopic samples are needed to understand whether the color excess is due exclusively to dust-obscured star formation, as well as the role of environment in triggering it.