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We present deep 3.6-8 μm imaging of the Hubble Deep Field-South with the Infrared Array Camera (IRAC) on the Spitzer Space Telescope. We study distant red galaxies (DRGs) at z > 2 selected by Js-Ks > 2.3 and compare them with a sample of Lyman break galaxies (LBGs) at z = 2-3. The observed UV-to-8 μm spectral energy distributions are fitted with stellar population models to constrain star formation histories and derive stellar masses. We find that 70% of the DRGs are best described by dust-reddened star-forming models and 30% are very well fitted with old and "dead" models. Using only the I-Ks and Ks-4.5 μm colors, we can effectively separate the two groups. The dead systems are among the most massive at z ∼ 2.5 (mean stellar mass 〈M*〉 = 0.8 × 1011 M⊙) and likely formed most of their stellar mass at z > 5. To a limit of 0.5 × 1011 M⊙, their number density is ∼10 times lower than that of local early-type galaxies. Furthermore, we use the IRAC photometry to derive rest-frame near-infrared J, H, and K fluxes. The DRGs and LBGs together show a large variation (a factor of 6) in the rest-frame K-band mass-to-light ratios (M/LK), implying that even a Spitzer 8 μm-selected sample would be very different from a mass-selected sample. The average M/LK of the DRGs is about 3 times higher than that of the LBGs, and DRGs dominate the high-mass end. The M/LK values and ages of the two samples appear to correlate with derived stellar mass, with the most massive galaxies being the oldest and having the highest mass-to-light ratios, similar to what is found in the low-redshift universe.
Labbé et al. (Fri,) studied this question.