2006ApJ...644..792R

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_GR) 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_5–8.5µm) 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_5–8.5µmprovides a reliable estimate of L_IRfor most star-forming galaxies at z∼2. We show that the range of L_IRin the optical/near-IR-selected samples considered extends from ≃10¹⁰ to >10¹² L_☉, with a mean <L_IR≥∼2x10¹¹ L_☉. Using 24 µm observations as an independent probe of dust extinction, we find that, as in the local universe, the obscuration L_IR/L₁₆₀₀is 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_bolat z∼2 than at z∼0. We show that the values of L_IRand obscuration inferred from the UV spectral slope β generally agree well with the values inferred from L_5–8.5µmfor L_bol<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.