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

Abstract

We present a candidate sample of luminous bulges (including ellipticals) found within the Groth Strip Survey (GSS), with spectroscopic redshifts of 0.73 < z < 1.04 from the Keck Telescope. This work is distinguished by its use of two-dimensional two-component decomposition photometry from Hubble Space Telescope (HST) images to separate the bulge from any disk before applying the sample selection and to measure disk-free colors. We define a statistically complete sample of 86 bulges with r 1/4 profiles and luminosities brighter than I AB = 24. Although larger samples of distant early-type galaxies exist, this is the largest and most homogeneous sample of bulges at z ∼ 1 with spectroscopy. A brighter subset of 52 objects with added structural constraints defines our "quality sample" that is used to explore bulge luminosities and colors. We find that 85% of luminous (M B < 0) bulge candidates are present, but only as a minor (8%) population. In general, such candidates have luminosities and surface brightnesses lower than that of the very red bulges; have large disk fractions by luminosity; and have emission line widths typically less than 100 km s -1 . These properties are all inconsistent with those predicted for star-forming progenitors of the luminous bulges of today, i.e., the blue photobulges are not genuine blue ellipticals or bulges. Moreover, over 60% of the bulge candidates that are not very red appear to reside in galaxies with morphologies suggestive of interactions and mergers. Thus, our deeper, more extensive, and less disk-contaminated observations challenge prior claims by other groups that 30%-50% of field bulges or ellipticals are in a blue, star-forming phase at redshifts z < 1. We conclude, with the caveat that luminous ellipticals and bulges at z ∼ 1 have r 1/4 light profiles, that they, as do luminous early-type cluster galaxies at the same redshift, are already dominated by metal-rich, old stellar populations that have been fading from a formation epoch earlier than redshift z > 1.5. Only small amounts of residual star formation are needed to explain both the absence of bluening of bulges to today and the presence of emission lines seen in the Keck spectra of the very red distant galaxies. © 2005. The American Astronomical Society. All rights reserved.