2015ApJ...809...95H

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 1, the population matches the massive end of the Hubble sequence, with 30% of pure spheroids, 50% of galaxies with equally dominant disk and bulge components, and 20% of disks. At z∼2-3 however, there is a majority of irregular systems (∼60%-70%) with still 30% of pure spheroids. We then analyze the stellar populations, star formation rates (SFRs), gas fractions, and structural properties for the different morphologies independently. Our results suggest two distinct channels for the growth of bulges in massive galaxies. Around ∼30%-40% were already bulges at z∼2.5, with low average SFRs and gas fractions (10%-15%), high Sersic indices (n 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.