The DEEP Groth Strip Survey. VIII. The Evolution of Luminous Field Bulges at Redshift z ∼ 1

We present a sample of over 50 luminous field bulges (including ellipticals)found in the Groth Strip Survey (GSS), with 0.73< z < 1.04 and with bulgemagnitudes I <= 23. The exponential disk light is removed via decomposition ofHST images using GIM2D. We find that 85% of these bulges are nearly as red aslocal E/S0's and have a shallow slope and a small color dispersion in thecolor-luminosity relation, suggesting roughly coeval formation. The surfacebrightnesses of these bulges are about 1 mag higher than local bulges. Theseresults are explained adopting a "drizzling" scenario where a metal-rich earlyformation is later polluted by small amounts of additional star formation.Almost all disks have the same or bluer colors than their accompanying bulges,regardless of the bulge-disk ratio and bulge luminosity, as expected fromsemi-analytic hierarchical galaxy formation models. We present evidence thatthe few blue bulge candidates are not likely to be genuine blue ellipticals orbulges. Our deeper, more extensive, and less disk-contaminated observationschallenge prior claims that 30% to 50% of field bulges or ellipticals are in ablue, star-forming phase at z < 1. We conclude that field bulges andellipticals at z ~ 1, like luminous early- type cluster galaxies at the sameredshift, are already dominated by metal-rich, old stellar populations thathave been fading from a formation epoch earlier than z ~ 1.5. (abridged)