2008ApJ...688..885H

We have combined data from the NOAO Deep Wide-Field Survey in Boötes and the Spitzer Space Telescope to determine basic properties for 16 optically ``invisible'' MIPS 24 µm and 35 optically invisible radio sources (OIMSs and OIRSs, respectively), including their spectral energy distributions (SEDs) and luminosities. Most OIMSs possess steep power-law SEDs over λ_rest=1-10 µm, indicating the presence of obscured AGNs in agreement with Spitzer spectroscopy. These objects are extremely luminous at rest-frame near- and mid-IR [νL_ν(5 µm)~10³⁸-10³⁹ W], consistent with accretion near the Eddington limit and further implying that they host buried QSOs. The majority of the IRAC-detected OIRSs have flat 3.6-24 µm SEDs, implying comparable emission from stellar photospheres and hot AGN-illuminated dust. This may reflect relatively small amounts of dust close to the central engine or current low mass accretion rates. A small subset of OIRSs appear to be starburst-dominated with photometric redshifts from 1.0 to 4.5. The OIMSs and OIRSs with significant starburst components have similar L_K and stellar masses (M_*~10¹¹ M_☉) assuming minimal AGN contribution. Roughly half of the OIRSs are not detected by Spitzer IRAC or MIPS. These are most likely z≳2 radio galaxies. The IRAC-detected OIRSs are more likely than OIMSs to appear nonpointlike in the 3.6 and 4.5 µm images, suggesting that interactions play a role in triggering their activity. The AGN-powered OIMSs may represent submillimeter galaxies making the transition from starburst to accretion dominance in their evolution to current-epoch massive ellipticals.