Spitzer Observations of Red Galaxies: Implication for High–Redshift Star Formation

My colleagues and I identified distant red galaxies (DRGs) with J −Ks>2.3 in the southern Great Observatories Origins Deep Surveys (GOODS–S) field. These galaxies reside at z∼1–3.5, (〈z〉≃2.2) and based on their ACS (0.4–1 μm), ISAAC (1–2.2 μm), and IRAC (3–8 μm) photometry, they typically have inferred stellar masses M&10 M⊙. Interestingly, more than 50% of these objects have 24 μm flux densities ≥50 μJy. Attributing the IR emission to star–formation implies star– formation rates (SFRs) of ≃100–1000 M⊙ yr . As a result, galaxies with M≥ 10 M⊙ have specific SFRs equal to or exceeding the global value at z∼1.5–3. In contrast, galaxies with M≥10 M⊙ at z∼0.3–0.75 have specific SFRs less than the global average, and more than an order of magnitude lower than that for massive DRGs at z∼1.5–3. Thus, the bulk of star formation in massive galaxies is largely complete by z∼1.5. The red colors and large inferred stellar masses in the DRGs suggest that much of the star formation in these galaxies occurred at redshifts z&5– 6. Using model star–formation histories that match the DRG colors and stellar masses at z∼2–3, and measurements of the UV luminosity density at z&5–6, we consider what constraints exist on the stellar initial mass function in the progenitors of the massive DRGs at z∼2–3.