Star Formation and Extinction in Redshiftz∼2 Galaxies: Inferences fromSpitzerMIPS Observations

We use very deep Spitzer MIPS 24 μm observations to examine the bolometric luminosities ( L bol ) and UV extinction properties of more than 200 spectroscopically identified, optically selected ( U n G ) 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 ( L 5-8.5 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 L 5-8.5 μm provides a reliable estimate of L IR for most star-forming galaxies at z ~ 2. We show that the range of L IR in the optical/near-IR-selected samples considered extends from ≃10 10 to >10 12 L ☉ , with a mean ⟨ L IR ⟩ ≃ 2 × 10 11 L ☉ . Using 24 μm observations as an independent probe of dust extinction, we find that, as in the local universe, the obscuration L IR / L 1600 is strongly dependent on L bol and ranges in value from <1 to ~1000 within the sample considered. However, the obscuration is generally ~10 times smaller at a given L bol at z ~ 2 than at z ~ 0. We show that the values of L IR and obscuration inferred from the UV spectral slope β generally agree well with the values inferred from L 5-8.5 μm for L bol < 10 12 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.