Spitzer Observations of MAMBO Galaxies: Weeding Out Active Nuclei in Starbursting Protoellipticals

We present Spitzer observations in five wavebands between 3.6 and 24um of anunbiased sample of 9 luminous, dusty galaxies selected at 1200um by the MAMBOcamera on the IRAM 30-m telescope, a population akin to the well-known submm or`SCUBA' galaxies (hereafter SMGs). Owing to the coarse resolution of submm/mminstrumentation, SMGs have traditionally been difficult to identify at otherwavelengths. We compare our multi-wavelength catalogs to show that the overlapbetween 24 and 1200um must be close to complete at these flux levels. We findthat all (4/4) of the most secure >=4sigma SMGs have robust >=4sigmacounterparts at 1.4GHz, while the fraction drops to 7/9 using all >=3sigmaSMGs. We show that combining mid-IR and marginal (>=3sigma) radio detectionsprovides plausible identifications in the remaining cases, enabling us toidentify the complete sample. Accretion onto an obscured central engine isbetrayed by the shape of the mid-IR continuum emission for several sources,confirming Spitzer's potential to weed out active galaxies. We demonstrate thepower of a S(24um)/S(8um) vs S(8um)/S(4.5um) color-color plot as a diagnosticfor this purpose. However, we conclude that the majority (~75%) of SMGs haverest-frame mid-/far-IR SEDs commensurate with obscured starbursts. Sensitive24-um observations are clearly a useful route to identify and characterizereliable counterparts to high-redshift far-IR-bright galaxies, complementingwhat is possible via deep radio imaging.