Irac Mid-infrared Imaging of the Hubble Deep Field South: Star Formation Histories and Stellar Masses of Red Galaxies

We present deep 3.6 − 8μm imaging of the Hubble Deep Field South with IRAC on the Spitzer Space Telescope. We study Distant Red Galaxies (DRGs) at z > 2 selected by Js − Ks > 2.3 and compare them to a sample of Lyman Break Galaxies (LBGs) at z = 2 − 3. The observed UV-to-8μm spectral energy distributions are fit 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 fit with old and “dead” models. Using only the I − Ks and Ks − 4.5μm 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× 10M⊙) and likely formed most of their stellar mass at z > 5. To a limit of 0.5× 10M⊙ their number density is ∼ 10× 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 8μm−selected sample would be very different from a mass-selected sample. The average M/LK of the DRGs is about three times higher than that of the LBGs, and DRGs dominate the high-mass end. The M/LK ratios 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 as found in the low-redshift universe. Subject headings: galaxies: evolution — galaxies: high redshift — infrared: galaxies