JWST/MIRI unveils the stellar component of the GN20 dusty galaxy overdensity at z=4.05

Dusty star-forming galaxies (DSFGs) at z > 2 have been commonly observed in overdense regions, where the merging processes and large halo masses induce rapid gas accretion, triggering star formation rates (SFRs) up to similar to 1000 M-circle dot yr(-1). Despite the importance of these DSFGs for understanding star formation in the early Universe, their stellar distributions, traced by the near-infrared (near-IR) emission, had remained spatially unresolved until the arrival of the JWST. In this work, we present, for the first time, a spatially resolved morphological analysis of the rest-frame near-IR (similar to 1.1-3.5 mu m) emission in DSFGs traced with the JWST/MIRI F560W, F770W, F1280W, and F1800W filters. In particular, we studied the mature stellar component for the three DSFGs and a Lyman-break galaxy (LBG) present in an overdensity at z = 4.05. Moreover, we used these rest-frame near-IR images along with ultraviolet (UV) and (sub)-mm ancillary photometric data to model their spectral energy distributions (SEDs) and extract their main physical properties (e.g. M*, SFR, A(V)). The sub-arcsec resolution images from the JWST have revealed that the light distributions in these galaxies present a wide range of morphologies, from disc-like to compact and clump-dominated structures. Two DSFGs and the LBG are classified as late-type galaxies (LTGs) according to non-parametric morphological indices, while the remaining DSFG is an early-type galaxy (ETG). These near-IR structures contrast with their ultraviolet emission, which is diffuse and, in GN20 and GN20.2b, off-centred by similar to 4 kpc. This result suggests that star formation takes place across the entire galaxy, while the UV light traces only those regions where the otherwise high internal extinction decreases significantly. The SED fitting analysis yields large SFRs (similar to 300-2500 M-circle dot yr(-1)), large stellar masses (M* = (0.24-1.79) x 1011 M-circle dot), and high integrated extinction values (AV = 0.8-1.5 mag) for our galaxies. In particular, we observe that GN20 dominates the total SFR with a value 2550 +/- 150 M-circle dot yr(-1), while GN20.2b has the highest stellar mass (M* = (2.2 +/- 1.4) x 1011 M-circle dot). The two DSFGs classified as LTGs (GN20 and GN20.2a) have a high specific SFR (sSFR > 30 Gyr(-1)), placing them above the star-forming main sequence (SFMS) at z similar to 4 by similar to 0.5 dex; whereas the ETG (i.e. GN20.2b) is compatible with the high-mass end of the main sequence. In comparison with other DSFGs in overdensities at z similar to 2-7, we observe that our objects present similar SFRs, depletion times, and projected separations. Nevertheless, the sizes computed for GN20 and GN20.2a are up to two times larger than those of isolated galaxies observed in CEERS and ALMA-HUDF at similar redshifts. We interpret this difference in size as an effect of rapid growth induced by the dense environment.