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

Using very deep Spitzer/MIPS 24 micron observations, we present an analysisof the bolometric luminosities (L[bol]) and UV extinction properties of morethan 200 spectroscopically identified, optically selected (UGR) z~2 galaxies inthe GOODS-N field. The large spectroscopic sample is supplemented with near-IRselected (BzK/DRG) galaxies and submm sources at similar redshifts in the samefield, providing a representative collection of relatively massive (M*>1e10Msun) galaxies at high redshifts. We focus on the redshift range 1.5-2.6, whereMIPS is sensitive to the strength of the mid-IR PAH features in the galaxyspectra (rest-frame 5-8.5 micron). We demonstrate, using stacked X-ray data anda subset of galaxies with H-alpha measurements, that L(5-8.5) provides areliable estimate of L(IR) for most star forming galaxies at z~2. The range ofL(IR) in the samples considered extends from ~1e10 to >1e12 Lsun, with a meanof 2e11 Lsun. Using 24 micron observations to infer dust extinction in highredshift galaxies, 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. However, the obscuration is ~10 times smaller at a given L(bol) at z~2than at z=0. We show that the values of L(IR) and obscuration inferred from theUV spectral slope beta generally agree well with the values inferred fromL(5-8.5) for L(bol)<1e12 Lsun. For galaxies with L(bol)>1e12 Lsun, it is commonfor UV-based estimates to underpredict L(IR) by a factor of ~10-100. Using thespecific SFR as a proxy for cold gas fraction, we find a wide range in theevolutionary state of galaxies at z~2, from those which have just begun to formstars to those which have already accumulated most of their stellar mass andare about to become, or already are, passively-evolving. [Abridged]Comment: 24 pages, 15 figures, 3 tables; accepted for publication in Ap