The nature of growing bulges within z < 1.3 galaxy disks in the GOODS-N
  field
  We analyze central surface brightness mu_0, nuclear and global colors of
intermediate redshift disk galaxies. On an apparent-diameter limited sample of
398 galaxies from ACS/HST 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 mu_0 only; all
high-mu_0 nuclei 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 ~ 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 AGNs become
common at 0.8 < z < 1.3, perhaps pointing to a role of AGN in the growth or
star formation truncation of bulges.
