Spitzer Observations of Massive, Red Galaxies at High Redshift**This work is based in part on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under NASA contract 1407; on observations taken with the NASA/ESA Hubble Space Telescope, which is operated by the Association of Universities for Research in Astronomy (AURA), Inc., under NASA contract NAS5-26555; and on observations collected at the Kitt Peak National Observatory (KPNO), National Optical Astronomical Observatory (NOAO), which is operated by AURA, Inc., under cooperative agreement with the National Science Foundation. Observations have also been carried out using the Very Large Telescope at the ESO Paranal Observatory under program ID LP168.A-0485.
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We study massive galaxies at z 1-3.5 using HST optical imaging, ground-based near-IR imaging, and Spitzer observations at 3-24 m. From Ks-selected galaxies in the 130 arcmin2 GOODS-S field, we identify 153 distant red galaxies (DRGs) with (J - Ks) Vega 2.3. This sample is approximately complete in stellar mass for passively evolving galaxies above 1011 M and z 3. Roughly half of the DRGs are objects whose optical and near-IR rest-frame light is dominated by evolved stars combined with ongoing star formation (at zmed 2.5), and the others are galaxies whose light is dominated by heavily reddened (A1600 4-6 mag) starbursts (at zmed 1.7). Very few DRGs ( 10%) have no indication of current star formation. DRGs at z 1.5-3 with stellar masses 1011 M have specific star formation rates (SFRs per unit mass) including the reradiated far-IR emission that range from 0.2 to 10 Gyr-1. Based on the X-ray luminosities and rest-frame near-IR colors, roughly one-quarter of the DRGs contain AGNs, implying that the growth of supermassive black holes coincides with the formation of massive galaxies. At 1.5 z 3, the DRGs with M 1011 M have an integrated specific SFR comparable to the global value of all galaxies. In contrast, galaxies at z 0.3-0.75 with M 1011 M have an integrated specific SFR less than the global value and more than an order of magnitude lower than that for massive DRGs. At z 1, lower mass galaxies dominate the overall cosmic mass assembly. This suggests that the bulk of star formation in massive galaxies occurs at early cosmic epochs and is largely complete by z 1.5. Further mass assembly in these galaxies takes place with low specific SFRs. 2006. The American Astronomical Society. All rights reserved.
