The Relative Star Formation Histories of Spiral Bulges and Elliptical Galaxies in the Hubble Deep Fields

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 I814 < 24 mag early-type and 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 colors of ellipticals in our sample. Comparisons are possible to a redshift z ≃ 1, at which point well-defined bulges become difficult to locate. The reliability of determining bulge colors using central apertures is tested by considering the homogeneity of the pixel-by-pixel colors for typical cases and through comparisons based on the simulated appearance at moderate redshift of the local sample of de Jong. We show via these tests and by selecting HDF subsets chosen according to inclination that disk contamination effects should be minimal. 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, particularly those at intermediate redshifts. We examine the optical and near-infrared colors of the HDF bulges in the context of models that include the effects of secondary star formation superimposed 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 particularly prevalent at z ≃ 0.6 as the limited HDF data seems to imply.