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D.C. Koo (UCO-Lick), DEEP Collaboration
We study the evolution of bulges at redshift z ~ 0.8 by using HST WFPC2 photometry. Redshifts are from Keck spectra. After separating out the disks using GIM2D software, we find the field (Groth Survey Strip) and cluster (MS1054-03) bulges have virtually identical color-magnitude relations (CMR). The CMR shows very red (U-B ~ 0.5) colors, a shallow slope, and small color scatter. These results are most consistent with an early epoch of rapid star formation for nearly all bulges and are inconsistent with theories in which the bulge colors of clusters and field differ substantially at redshifts z ~ 0.8 from each other or from those seen today. Our results disagree with several prior claims for 30% to 50% blue spheroids or spheroidals by redshifts z ~ 1. We do, however, find evidence for continued, but tiny amounts of, star formation in galaxies in which both the bulge and disk components are very red.
This work has been supported by NASA and NSF.
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Bulletin of the American Astronomical Society, 35#5
© 2003. The American Astronomical Soceity.