Spitzer observations of MAMBO galaxies:: Weeding out active nuclei in starbursting protoellipticals

We present 3.6 - 24 mum Spitzer observations of an unbiased sample of nine luminous, dusty galaxies selected at 1200 mum by MAMBO on the IRAM 30 m telescope, a population akin to the well-known submillimeter or SCUBA galaxies ( hereafter SMGs). Owing to the coarse resolution of submillimeter/millimeter cameras, SMGs have traditionally been difficult to identify at other wavelengths. We compare our multiwavelength catalogs to show that the overlap between 24 and 1200 mum must be close to complete at these flux levels. We find that all (4/4) of the most secure greater than or equal to4 sigma SMGs have greater than or equal to 4 sigma counterparts at 1.4 GHz, while the fraction drops to 7/9 using all greater than or equal to 3 sigma SMGs. We show that combining mid-infrared (MIR) and marginal ( greater than or equal to 3 sigma) radio detections provides plausible identifications in the remaining cases, enabling us to identify the complete sample. Accretion onto an obscured central engine is betrayed by the shape of the MIR continuum emission for several sources, confirming Spitzer's potential to weed out active galaxies. We demonstrate the power of an S-24 mum/S-8 mum versus S-8 mum/S-4.5 mum color-color plot as a diagnostic for this purpose. However, we conclude that the majority ( similar to 75%) of SMGs have rest-frame mid/far-IR spectral energy distributions commensurate with obscured starbursts. Sensitive 24 mum observations are clearly a useful route to identify and characterize reliable counterparts to high-redshift far-IR-bright galaxies, complementing what is possible via deep radio imaging.