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

Dusty star-forming galaxies (DSFGs) atz>2 have been commonly observed in overdense regions, where the merging processes and large halomasses  induce  rapid  gas  accretion,  triggering  star-formation  rates  (SFRs)  up  to∼1000M⊙yr−1.  Despite  the  importance  of  these  DSFGs  forunderstanding the star-formation in the early Universe, their stellar distributions traced by the near-IR emission were spatially unresolved until thearrival of the JWST. In this work we present, for the first time, a spatially-resolved morphological analysis of the rest-frame near-IR (∼1.1−3.5μm)emission in DSFGs traced with the JWST/MIRI F560W, F770W, F1280W and F1800W filters. In particular, we study the mature stellar componentfor the three DSFGs and a Lyman-break galaxy (LBG) present in an overdensity atz=4.05. Moreover, we use these rest-frame near-IR imagesalong with ultraviolet (UV) and (sub)-mm ancillary photometric data to model their spectral energy distributions (SEDs) and extract their mainphysical properties (e.g.,M∗, SFR,AV). The sub-arcsec resolution images from the JWST have revealed that the light distributions in these galaxiespresent a wide range of morphologies, from disc-like to compact and clump-dominated structures. Two DSFGs and the LBG are classified aslate-type galaxies (LTGs) according to non-parametric morphological indices, while the remaining DSFG is an early-type galaxy (ETG). Thesenear-IR structures contrast with their UV emission, which is diffuse and, in GN20 and GN20.2b, off-centred by∼4 kpc. This result suggeststhat the star-formation occurs across the entire galaxy, while the UV light traces only those regions where the otherwise high internal extinctiondecreases  significantly.  The  SED  fitting  analysis  yields  large  SFRs  (∼300−2500M⊙yr−1),  large  stellar  masses  (M∗=(0.24−1.79)×1011M⊙)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 avalue 2550±150M⊙yr−1while GN20.2b has the highest stellar mass (M∗=(2.2±1.4)×1011M⊙). The two DSFGs classified as LTGs (GN20 andGN20.2a) have high specific SFR (sSFR>30 Gyr−1) placing them above the star-forming main sequence (SFMS) at z∼4 by∼0.5 dex, while theETG (i.e., GN20.2b) is compatible with the high-mass end of the main sequence. When comparing with other DSFGs in overdensities atz∼2−7,we observe that our objects present similar SFRs, depletion times and projected separations. Nevertheless, the sizes computed for GN20 andGN20.2a are up to two times larger than those of isolated galaxies observed in CEERS and ALMA-HUDF at similar redshifts. We interpret thisdifference in size as an effect of rapid growth induced by the dense environment.