Irac mid-infrared imaging of the Hubble Deep Field-South:: Star formation histories and stellar masses of red galaxies at z>2

We present deep 3.6 - 8 mu m 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 1 2.3 and compare them with a sample of Lyman break galaxies (LBGs) at z = 2 - 3. The observed UV - to - 8 mu m 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-s and K-s - 4.5 mu m colors, we can effectively separate the two groups. The dead systems are among the most massive at z similar to 2.5 ( mean stellar mass [M*] = 0.8 x 10(11) M-.) and likely formed most of their stellar mass at z > 5. To a limit of 0.5 x 10(11) M-., their number density is similar to 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 mu m-selected sample would be very different from a mass-selected sample. The average M/L-K of the DRGs is about 3 times higher than that of the LBGs, and DRGs dominate the high-mass end. The M/L-K 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.