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Abstract

The formation and assembly process of massive galaxies is a combination of two phases: an initial in-situ-dominated one followed by an ex-situ- dominated evolution. Separating these two contributions is therefore crucial to understand the baryonic cycle within massive haloes. A recently discovered population of so-called naked red nuggets, galaxies that shortcut the ex-situ stage preserving their pristine properties, presents a unique opportunity to study in detail star formation in massive galaxies without the confounding effect of later accretion. We investigate the spatially resolved star formation histories of a sample of 12 naked red nuggets. We measure how their radial light distributions, star formation rates, and central densities evolved in time. We find that, while forming stars, red nuggets become gradually more concentrated, reaching a maximum concentration at quenching. After being quenched, they kept forming stars in a more discy-like configuration. Our measurements suggest that supermassive black holes and host galaxies grow their mass in a self-regulated way until a characteristic M_∙/M_halo is reached. Once black holes are massive enough, red nuggets get quenched and depart from the star formation main sequence. While in the main sequence, red nuggets evolve at roughly constant star formation rate. This can explain up to ̃0.3 dex of the scatter of the star formation main sequence, as well as its higher normalization observed in the early Universe. Hence, our results suggest that the main sequence is composed of populations of galaxies at different evolutionary stages and that the scatter is therefore due to secular processes.