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We present the AGN, star-forming, and morphological properties of a sample of13 MIR-luminous (f(24) > 700uJy) IR-bright/optically-faint galaxies (IRBGs,f(24)/f(R) > 1000). While these z~2 sources were drawn from deep Chandra fieldswith >200 ks X-ray coverage, only 7 are formally detected in the X-ray and fourlack X-ray emission at even the 2 sigma level. Spitzer IRS spectra, however,confirm that all of the sources are AGN-dominated in the mid-IR, although halfhave detectable PAH emission responsible for ~25% of their mid-infrared fluxdensity. When combined with other samples, this indicates that at least 30-40%of luminous IRBGs have star-formation rates in the ULIRG range (~100-2000Msun/yr). X-ray hardness ratios and MIR to X-ray luminosity ratios indicatethat all members of the sample contain heavily X-ray obscured AGN, 80% of whichare candidates to be Compton-thick. Furthermore, the mean X-ray luminosity ofthe sample, log L(2-10 keV)(ergs/s)=44.6, indicates that these IRBGs are Type 2QSOs, at least from the X-ray perspective. While those sources most heavilyobscured in the X-ray are also those most likely to display strong silicateabsorption in the mid-IR, silicate absorption does not always accompany X-rayobscuration. Finally, ~70% of the IRBGs are merger candidates, a rateconsistent with that of sub-mm galaxies (SMGs), although SMGs appear to bephysically larger than IRBGs. These characteristics are consistent with theproposal that these objects represent a later, AGN-dominated, and more relaxedevolutionary stage following soon after the star-formation-dominated onerepresented by the SMGs.

The AGN, Star-Forming, and Morphological Properties of Luminous IR-Bright/Optically-Faint Galaxies