2009ApJ...694L..69D

We analyze central surface brightness µ₀, nuclear and global colors of intermediate redshift disk galaxies. On an apparent-diameter-limited sample of 398 galaxies from ACS/HST (Advanced Camera for Surveys in the Hubble Space Telescope) The Great Observatories Origins Deep Survey North (GOODS-N), we find 131 galaxies with bulges and 214 without. Up to z ∼ 0.8, blue, star-forming nuclei are found in galaxies with low µ₀only; all high-µ₀nuclei show red, passive colors, so that nuclear and global (U - B) colors strongly correlate with central surface brightness, as found in the local universe. At 0.8 < z < 1.3, a fraction of ∼27% of the high surface brightness nuclei show blue colors, and positive nuclear color gradients. The associated nuclear star formation must lead to bulge growth inside disks. Population modeling suggests that such blue bulges evolve into local pseudobulges rather than classical bulges. We do not find evidence for rejuvenation of classical bulges at the sampled z. High luminosity active galactic nuclei (AGNs) become common at 0.8 < z < 1.3, perhaps pointing to a role of AGNs in the growth or star formation truncation of bulges.