2005ApJ...624L..81L

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 J_s-K_s>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-K_sand K_s_-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_*>= 0.8x10¹¹ M_☉) and likely formed most of their stellar mass at z>5. To a limit of 0.5x10¹¹ 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/L_K), implying that even a Spitzer 8 µm-selected sample would be very different from a mass-selected sample. The average M/L_Kof the DRGs is about 3 times higher than that of the LBGs, and DRGs dominate the high-mass end. The M/L_Kvalues 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.