The morphologies of massive galaxies from z similar to 3-witnessingthe two channels of bulge growth

Abstract

We quantify the morphological evolution of z similar to 0 massive galaxies (M-*/M-circle dot similar to 10(11.2 +/- 0.3)) from z similar to 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 similar to 2-3 however, there is a majority of irregular systems (similar to 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 similar to 30%-40% were already bulges at z similar to 2.5, with low average SFRs and gas fractions (10%-15%), high Sersic indices (n > 3-4), and small effective radii (R-e similar to 1 kpc), pointing toward an even earlier formation through gas-rich mergers or violent disk instabilities. Between z similar to 2.5 and z similar to 0, they rapidly increase their size by a factor of similar to 4-5, are quenched, and slightly increase their Sersic indices (n similar to 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 similar to 3) for 2/3 of the systems, possibly through the migration of clumps, while the remaining 1/3 retain a rather small bulge (n similar to 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.