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Title:
Mid-Infrared Spectral Diagnosis of Submillimeter Galaxies
Authors:
Pope, Alexandra; Chary, Ranga-Ram; Alexander, David M.; Armus, Lee; Dickinson, Mark; Elbaz, David; Frayer, David; Scott, Douglas; Teplitz, Harry
Affiliation:
AA(Department of Physics and Astronomy, University of British Columbia, Vancouver, BC V6T 1Z1, Canada.; Current address: National Optical Astronomy Observatory, Tucson, AZ 85719; .; Spitzer Fellow.), AB(Spitzer Science Center, MS 220-6, California Institute of Technology, Pasadena, CA 91125.), AC(Department of Physics, Durham University, Durham DH1 3LE, UK.), AD(Spitzer Science Center, MS 220-6, California Institute of Technology, Pasadena, CA 91125.), AE(National Optical Astronomy Observatory, Tucson, AZ 85719.), AF(Laboratoire AIM, CEA/DSM-CNRS-Université Paris Diderot, DAPNIA/Service d'Astrophysique, CEA Saclay, 91191 Gif-sur-Yvette Cedex, France.), AG(Spitzer Science Center, MS 220-6, California Institute of Technology, Pasadena, CA 91125.), AH(Department of Physics and Astronomy, University of British Columbia, Vancouver, BC V6T 1Z1, Canada.), AI(Spitzer Science Center, MS 220-6, California Institute of Technology, Pasadena, CA 91125.)
Publication:
The Astrophysical Journal, Volume 675, Issue 2, pp. 1171-1193. (ApJ Homepage)
Publication Date:
03/2008
Origin:
UCP
ApJ Keywords:
Galaxies: Active, Galaxies: Evolution, Galaxies: Starburst, Infrared: Galaxies, Submillimeter, Techniques: Spectroscopic
DOI:
10.1086/527030
Bibliographic Code:
2008ApJ...675.1171P

Abstract

We present deep mid-IR spectroscopy with Spitzer of 13 SMGs in the GOODS-N field. We find strong 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 AGN activity to the mid-IR emission. In the high-S/N composite spectrum, 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-IR 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 24 μm-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 SEDs to determine the best estimate of the total IR luminosity from star formation. While many SMGs contain an AGN as evinced by their X-ray properties, our multiwavelength analysis shows that the total IR luminosity, LIR, in SMGs is dominated by star formation. We find that high-redshift SMGs lie on the relation between LIR and LPAH,6.2 (or LPAH,7.7 or LPAH,11.3) that has been established for local starburst galaxies. This suggests that PAH luminosity can be used as a proxy for the SFR in SMGs. SMGs are consistent with being a short-lived cool phase in a massive merger where the AGN does not appear to have become strong enough to heat the dust and dominate the mid- or far-IR emission.
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