Mid-Infrared Spectral Diagnosis of Submillimeter Galaxies

We present deep mid-infrared spectroscopy with the Spitzer Space Telescope of 13 submillimeter galaxies (SMGs) in the Great Observatories Origins Deep Survey North (GOODS-N) field. We find strong polycyclic aromatic hydrocarbon (PAH) emission in all of our targets which allows us to measure mid-IR spectroscopic redshifts and place constraints on the contribution from star formation and active galactic nuclei (AGN) activity to the mid-IR emission. In the high signal-to-noise ratio composite spectrum of SMGs, we find that the hot dust continuum from an AGN contributes at most 30% of the mid-IR luminosity. Individually, only 2/13 SMGs have continuum emission dominating the mid-infrared luminosity; one of these SMGs, C1, remains undetected in the deep X-ray images but shows a steeply rising continuum in the mid-IR indicative of a Compton-thick AGN. We find that the mid-IR properties of SMGs are distinct from those of 24μm-selected ULIRGs at z ∼ 2; the former are predominantly dominated by star formation while the latter are a more heterogeneous sample with many showing significant AGN activity. We fit the IRS spectrum and the mid-IR to radio photometry of SMGs with template spectral energy distributions to determine the best estimate of the total infrared luminosity from star formation. While many SMGs contain an AGN as evinced by their X-ray properties, our multi-wavelength analysis shows that the total infrared luminosity, LIR, in SMGs, is dominated by star formation and not AGN activity. We Department of Physics & Astronomy, University of British Columbia, Vancouver, BC, V6T 1Z1, Canada Spitzer Science Center, MS 220-6 Caltech, Pasadena, CA 91125 Department of Physics, Durham University, Durham, DH1 3LE, UK National Optical Astronomy Observatory, 950 N. Cherry Ave., Tucson, AZ, 85719 Laboratoire AIM, CEA/DSM CNRS Université Paris Diderot, DAPNIA/Service d’Astrophysique, CEA Saclay, 91191 Gif-sur-Yvette Cedex, France