Star formation and extinction in redshift z∼2 galaxies:: Inferences from Spitzer MIPS observations

We use very deep Spitzer MIPS 24 mu m observations to examine the bolometric luminosities (L-bol) and UV extinction properties of more than 200 spectroscopically identified, optically selected (U(n)GR) z similar to 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 mu m observations measure the strength of the mid-IR PAH feature, we find that the rest-frame 5-8.5 mu m luminosities (L5-8.5 mu m) 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 H alpha measurements, that L5-8.5 mu m provides a reliable estimate of L-IR for most star-forming galaxies at z similar to 2. We show that the range of L-IR in the optical/near-IR-selected samples considered extends from similar or equal to 10(10) to > 10(12) L-circle dot, with a mean < L-IR > similar or equal to 2 x 10(11) L-circle dot. Using 24 mu m 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 similar to 1000 within the sample considered. However, the obscuration is generally similar to 10 times smaller at a given Lbol at z similar to 2 than at z similar to 0. We show that the values of L-IR and obscuration inferred from the UV spectral slope beta generally agree well with the values inferred from L5-8.5 mu m for L-bol < 10(12) L-circle dot. 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 similar to 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.