Spitzer observations of red galaxies: Implication for high-redshift star formation

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My colleagues and I identified distant red galaxies (DRGs) with J − K s > 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 stellar masses M ⩾ 10 11 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 −1. As a result, galaxies with M ⩾ 10 11 M ⊙ have specific SFRs equal to or exceeding the global value at z ∼ 1.5–3. In contrast, galaxies with M ⩾ 10 11 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.