  Hierarchical galaxy formation models make specific predictions concerning the
relative assembly rates and star formation histories of spiral bulges and field
ellipticals. Independently of the cosmological model and initial power spectrum
of fluctuations, at all epochs the stellar populations in spiral bulges should
be older and redder than those in typical ellipticals selected at the same
redshift. To test this simple prediction, we analyze the internal optical
colors of a complete sample of I<24 mag early-type and spiral galaxies from the
Northern and Southern Hubble Deep Fields (HDF). The subset of galaxies in the
Northern HDF are also investigated in the near-infrared using NICMOS
photometry. We compare the central (inner 5% radius) colors of those spirals
with clearly visible bulges with the integrated colors of ellipticals in our
sample. Comparisons are possible to a redshift z~1 at which point well-defined
bulges become difficult to locate. While spiral bulges are systematically
redder in their optical colors than their associated disks at all redshifts, we
find that the majority are significantly bluer than the red locus occupied by
most field ellipticals at similar redshifts. In the near-infrared, similar
trends are found at redshifts z<0.6, but at higher redshifts some bulges as red
as the reddest ellipticals are found. We conclude that a significant
rejuvenation may have occurred in the inner stellar populations of many spiral
galaxies at intermediate redshifts.
