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

R.J. Ivison, T.R. Greve, S. Serjeant, F. Bertoldi, E. Egami, A.M.J. Mortier, A. Alonso-Herrero, P. Barmby, L. Bei, H. Dole, C.W. Engelbracht, G.G. Fazio, D.T. Frayer, K.D. Gordon, D.C. Hines, J.-S. Huang, E. Le Floch, K.A. Misselt, S. Miyazaki, J.E. Morrison, C. Papovich, P.G. Perez-Gonzalez, M.J. Rieke, G.H. Rieke, J. Rigby, D. Rigopoulou, I. Smail, G. Wilson, S.P. Willner 2004, ApJS, 154, 124

We present Spitzer observations in five wavebands between 3.6 and 24 µm of an unbiased sample of 9 luminous, dusty galaxies selected at 1200um by the MAMBO camera 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/mm instrumentation, SMGs have traditionally been difficult to identify at other wavelengths. We compare our multi-wavelength catalogs to show that the overlap between 24 and 1200 µm must be close to complete at these flux levels. We find that all (4/4) of the most secure >e;4sigma SMGs have robust ≥4sigma counterparts at 1.4GHz, while the fraction drops to 7/9 using all ≥3sigma SMGs. We show that combining mid-IR and marginal (≥3sigma) 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 mid-IR continuum emission for several sources, confirming Spitzer's potential to weed out active galaxies. We demonstrate the power of a S(24µm)/S(8µm) vs S(8µm)/S(4.5µm) color-color plot as a diagnostic for this purpose. However, we conclude that the majority (~75%) of SMGs have rest-frame mid-/far-IR SEDs commensurate with obscured starbursts. Sensitive 24 µm 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.

[XXX/astro-ph Preprint] [ADS Entry]

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