  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 halos. 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 disky-like configuration. Our
measurements suggest that super-massive black holes and host galaxies grow
their mass in a self-regulated way until a characteristic Mbh/Mhalo 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.
