Star Formation and Extinction in Redshift Z ∼ 2 Galaxies: Inferences from Spitzer Mips Observations

Using very deep Spitzer MIPS 24 μm observations, we present an analysis of the bolometric luminosities and UV extinction properties of more than 200 spectroscopically identified, optically selected (UnGR) z ∼ 2 galaxies in the GOODS-N field. The large spectroscopic sample of rest-UV selected galaxies is supplemented with photometrically identified near-IR-selected (“BzK” and “DRG”) galaxies and sub-mm sources at similar redshifts in the same field, providing a representative collection of relatively massive (M > 10 M⊙) galaxies at high redshifts. We focus on the redshift range 1.5 < z < 2.6, for which the 24 μm observations provide a direct measurement of the strength of the mid-IR PAH features in the galaxy spectra; the rest-frame 5 − 8.5 μm luminosities (L5−8.5μm) are particularly tightly constrained for the objects in our sample with precise spectroscopic redshifts. We demonstrate, using stacked X-ray observations and a subset of galaxies with Hα measurements, that L5−8.5μm 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 ≃ 10 10 L⊙ to > 10 12 L⊙, with a mean 〈LIR〉 ≃ 2×10 11 L⊙. The LIRG population at z ∼ 2 is essentially the same population of galaxies that are selected by their optical/near-IR colors. Objects with LIRG to ULIRG luminosities are present over the full range of stellar masses in the samples, from 2× 10 M⊙ to 5× 10 11 M⊙. We use the MIPS 24 μm observations for an independent examination of dust extinction in high redshift galaxies, and demonstrate that, as in the local universe, the obscuration ( LIR L1600 ) is strongly dependent on bolometric luminosity, and ranges in value from < 1 to ∼ 1000 within the sample considered. However, the obscuration is ∼ 10 times smaller at a given Lbol (or, equivalently, a similar level of obscuration occurs at luminosities ∼ 10 times larger) 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 L5−8.5μm for Lbol < 10 12 L⊙. As found previously by several investigators, for “ultraluminous” objects with Lbol > 10 12 L⊙ it is common for UV-based estimates to underpredict LIR by a factor of ∼ 10− 100. Using the specific star formation rate of galaxies (SFR per unit stellar mass) 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 which have already accumulated most of their stellar mass and are about to become, or already are, passively-evolving. Subject headings: cosmology: observations — dust, extinction — galaxies: evolution — galaxies: high redshift — galaxies: stellar content — infrared: galaxies