Star Formation and Extinction in Redshift z~2 Galaxies: Inferences from Spitzer MIPS Observations

We use very deep Spitzer MIPS 24 μm observations to examine the bolometric luminosities (Lbol) and UV extinction properties of more than 200 spectroscopically identified, optically selected (UnGℛ) z ~ 2 galaxies, supplemented with near-IR-selected ("BzK" and "DRG") and submillimeter galaxies at similar redshifts, in the GOODS-N field. Focusing on redshifts 1.5 < z < 2.6, where 24 μm observations measure the strength of the mid-IR PAH feature, we find that the rest-frame 5-8.5 μm luminosities (L5-8.5 μm) are particularly tightly constrained for objects in our sample with precise spectroscopic redshifts. We demonstrate, using stacked X-ray observations and a subset of galaxies with Hα measurements, that L5-8.5 μm provides a reliable estimate of LIR for most star-forming galaxies at z ~ 2. We show that the range of LIR in the optical/near-IR-selected samples considered extends from ≃1010 to >1012 L☉, with a mean ⟨LIR⟩ ≃ 2 × 1011 L☉. Using 24 μm observations as an independent probe of dust extinction, we find that, as in the local universe, the obscuration LIR/L1600 is strongly dependent on Lbol and ranges in value from <1 to ~1000 within the sample considered. However, the obscuration is generally ~10 times smaller at a given Lbol at z ~ 2 than at z ~ 0. We show that the values of LIR and obscuration inferred from the UV spectral slope β generally agree well with the values inferred from L5-8.5 μm for Lbol < 1012 L☉. Using the specific SFRs of galaxies as a proxy for cold gas fraction, we find a wide range in the evolutionary state of galaxies at z ~ 2, from galaxies that have just begun to form stars to those that have already accumulated most of their stellar mass and are about to become, or already are, passively evolving.