The morphologies of massive galaxies from z ~ 3-witnessing the two channels of bulge growth

We quantify the morphological evolution of z ~ 0 massive galaxies (M_*/M_∼ ∼10^11.2 +/- 0.3) from z ~ 3 in the 5 CANDELS fields. The progenitors are selected using abundance matching techniques to account for the mass growth. At z 3-4), and small effective radii (R_e ∼1 kpc), pointing toward an even earlier formation through gas-rich mergers or violent disk instabilities. Between z ~ 2.5 and z ~ 0, they rapidly increase their size by a factor of ~ 4-5, are quenched, and slightly increase their Sersic indices (n ~ 5) but their global morphology remains unaltered. The structural evolution is independent of the gas fractions, suggesting that it is driven by ex situ events. The remaining 60% experience a gradual morphological transformation, from clumpy disks to more regular bulge +disk systems, essentially happening at z > 1. This results in the growth of a significant bulge component (n ~ 3) for 2/3 of the systems, possibly through the migration of clumps, while the remaining 1/3 retain a rather small bulge (n ~ 1.5-2). The transition phase between disturbed and relaxed systems and the emergence of the bulge is correlated with a decrease in the star formation activity and the gas fractions, suggesting a morphological quenching process as a plausible mechanism for the formation of these bulges.