Compact dust-obscured star formation and the origin of the galaxy bimodality

Context. The combined capabilities of the James Webb Space Telescope/Near Infrared Camera (NIRCam) and the Hubble Space Telescope/Advanced Camera for Surveys (ACS) instruments provide high-angular-resolution imaging from the ultraviolet to near-infrared (UV/NIR), offering unprecedented insight into the inner structure of star-forming galaxies (SFGs) even when they are shrouded in dust. In particular, it is now possible to spatially resolve and study a population of highly attenuated and massive red SFGs (RedSFGs) at z ∼ 4 in the rest-frame optical/near-infrared (optical/NIR). Given their significant contribution to the cosmic star formation rate density (SFRD) at z > 3, these RedSFGs are likely to be the progenitors of the massive (log( M * / M ⊙ ) > 10) and passive galaxies already in place at cosmic noon ( z ∼ 2). They therefore represent a crucial population that can help elucidate the mechanisms governing the transition from vigorous star formation to quiescence at high redshifts. Aims. We assembled a mass-complete sample of massive galaxies at z = 3 − 4 to study and compare the stellar mass, star formation rate (SFR), dust attenuation, and age spatial distributions of RedSFGs with those of quiescent galaxies (QGs) and more typical blue SFGs (BlueSFGs). Methods. We performed an injection-recovery procedure with galaxies of various profiles in the CEERS images to build a mass-complete sample of 188 galaxies with log( M * / M ⊙ ) > 9.6, which we classified into BlueSFGs, RedSFGs, and QGs. We performed a resolved spectral energy distribution (SED) fitting on the UV/NIR data to compute and compare the radial profiles of these three populations. Results. The RedSFGs fraction is systematically higher than that of QGs and both are seen to increase with stellar mass. Together, they account for more than 50% of galaxies with log( M * / M ⊙ ) > 10.4 at this redshift. This transition mass corresponds to the log( M * / M ⊙ )∼10.4 threshold, often referred to as the “critical mass”, which delineates the bimodality between BlueSFGs and QGs. We find that RedSFGs and QGs present similar stellar surface density profiles and that RedSFGs manifest a dust attenuation concentration that is significantly higher than that of BlueSFGs at all masses. This suggests that a path for a BlueSFG to become quiescent is through a major compaction event, triggered once the galaxy reaches a sufficient mass, leading to the in situ formation of a massive bulge. Conclusions. There is a bimodality between extended BlueSFGs and compact and strongly attenuated RedSFGs that have undergone a phase of major gas compaction. There is evidence that this early-stage separation is at the origin of the local bimodality between BlueSFGs and QGs, which we refer to as a “primeval bimodality”.