The relative star formation histories of spiral bulges and elliptical galaxies in the Hubble Deep Fields

Hierarchical galaxy formation models make specific predictions concerning t he relative assembly rates and star formation histories of spiral bulges an d field ellipticals. Independently of the cosmological model and initial po wer spectrum of fluctuations, at all epochs the stellar populations in spir al bulges should be older and redder than those in typical ellipticals sele cted at the same redshift. To test this simple prediction, we analyze the i nternal optical colors of a complete sample of I-814 < 24 mag early- type a nd spiral galaxies from the northern and southern Hubble Deep Fields (HDF). The subset of galaxies in the HDF-North 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 col ors of ellipticals in our sample. Comparisons are possible to a redshift z <similar or equal to> 1, at which point well-defined bulges become difficul t to locate. The reliability of determining bulge colors using central aper tures is tested by considering the homogeneity of the pixel-by-pixel colors for typical cases and through comparisons based on the simulated appearanc e at moderate redshift of the local sample of de Jong. We show via these te sts and by selecting HDF subsets chosen according to inclination that disk contamination effects should be minimal. While spiral bulges are systematic ally redder in their optical colors than their associated disks at all reds hifts, we find that the majority are significantly bluer than the red locus occupied by most field ellipticals at similar redshifts. In the near-infra red, 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 populatio ns of many spiral galaxies, particularly those at intermediate redshifts. W e examine the optical and near-infrared colors of the HDF bulges in the con text of models that include the effects of secondary star formation superim posed upon preexisting old populations and conclude the data is best fitted when this secondary activity is burstlike. We discuss the consequences for models of secular evolution in disks should these bursts have been particu larly prevalent at z <similar or equal to> 0.6 as the limited HDF data seem s to imply.