Spitzer observations of massive, red galaxies at high redshift

We study massive galaxies at z similar to 1-3.5 using HST optical imaging, ground-based near-IR imaging, and Spitzer observations at 3-24 mu m. From K-s-selected galaxies in the similar or equal to 130 arcmin(2) GOODS-S field, we identify 153 distant red galaxies ( DRGs) with (J - K-s)(Vega) >= 2.3. This sample is approximately complete in stellar mass for passively evolving galaxies above 10(11) M-circle dot 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 z(med) similar to 2: 5), and the others are galaxies whose light is dominated by heavily reddened (A(1600) greater than or similar to 4-6 mag) starbursts (at z(med) similar to 1.7). Very few DRGs (less than or similar to 10%) have no indication of current star formation. DRGs at z similar to 1.5-3 with stellar masses similar to 10(11) M-circle dot 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 >= 10(11) M-circle dot have an integrated specific SFR comparable to the global value of all galaxies. In contrast, galaxies at z similar to 0.3-0.75 with M >= 10(11) M-circle dot 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 P 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 similar to 1.5. Further mass assembly in these galaxies takes place with low specific SFRs.