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

Abstract

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.