Coppin, K. and Pope, A. and Menéndez-Delmestre, K. and Alexander, D. M. and Dunlop, J. S. and Egami, E. and Gabor, J. and Ibar, Edo and Ivison, R. J. and Austermann, J. E. and Blain, A. W. and Chapman, S. C. and Clements, D. L. and Dunne, L. and Dye, S. and Farrah, D. and Hughes, D. H. and Mortier, A. M. J. and Page, M. J. and Rowan-Robinson, M. and Scott, D. and Simpson, C. and Smail, Ian and Swinbank, A. M. and Vaccari, M. and Yun, M. S. (2010) Mid-infrared Spectroscopy of Candidate Active Galactic Nuclei-dominated Submillimeter Galaxies. Astrophysical Journal, 713 (1). pp. 503-519. ISSN 0004-637X http://resolver.caltech.edu/CaltechAUTHORS:20100415-110421934
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Spitzer spectroscopy has revealed that ≃80% of submm galaxies (SMGs) are starburst (SB)-dominated in the mid-infrared. Here we focus on the remaining ≃20% that show signs of harboring powerful active galactic nuclei (AGNs). We have obtained Spitzer-InfraRed Spectrograph spectroscopy of a sample of eight SMGs that are candidates for harboring powerful AGNs on the basis of IRAC color selection (S_(8 μm)/S_(4.5 μm) > 2, i.e., likely power-law mid-infrared spectral energy distributions). SMGs with an AGN dominating (≳50%) their mid-infrared emission could represent the "missing link" sources in an evolutionary sequence involving a major merger. First of all, we detect polycyclic aromatic hydrocarbon (PAH) features in all of the SMGs, indicating redshifts from 2.5 to 3.4, demonstrating the power of the mid-infrared to determine redshifts for these optically faint dusty galaxies. Second, we see signs of both star formation (from the PAH features) and AGN activity (from continuum emission) in our sample: 62% of the sample are AGN-dominated in the mid-infrared with a median AGN content of 56%, compared with <30% on average for typical SMGs, revealing that our IRAC color selection has successfully singled out sources with proportionately more AGN emission than typical SB-dominated SMGs. However, we find that only about 10% of these AGNs dominate the bolometric emission of the SMG when the results are extrapolated to longer infrared wavelengths, implying that AGNs are not a significant power source to the SMG population overall, even when there is evidence in the mid-infrared for substantial AGN activity. When existing samples of mid-infrared AGN-dominated SMGs are considered, we find that S_(8 μm)/S_(4.5 μm) > 1.65 works well at selecting mid-infrared energetically dominant AGNs in SMGs, implying a duty cycle of ~15% if all SMGs go through a subsequent mid-infrared AGN-dominated phase in the proposed evolutionary sequence.
|Additional Information:||© 2010 American Astronomical Society. Issue 1 (2010 April 10): received 2009 November 20; accepted for publication 2010 February 28. This work is based on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory (JPL), California Institute of Technology under a contract with NASA. The IRS was a collaborative venture between Cornell University and Ball Aerospace Corporation funded by NASA through the JPL and Ames Research Center. Support for this work was provided by NASA through an award issued by JPL/Caltech. This work is based in part on data obtained as part of the UKIRT Infrared Deep Sky Survey (UKIDSS). K.C. acknowledges support from a UK Science and Technology Facilities Council fellowship. A.P. acknowledges support provided by NASA through the Spitzer Space Telescope Fellowship Program, through a contract issued by the JPL, California Institute of Technology under a contract with NASA. K.M.D. is supported by an NSF Astronomy and Astrophysics Postdoctoral Fellowship under award AST-0802399. D.M.A. thanks the Royal Society and the Philip Leverhulme fellowship prize for generous support. We thank an anonymous referee for suggestions which improved the paper. We also thank Ranga Chary for providing the full SED template of Mrk 231 and the nuclear region of NGC 1068, and to Laura Hainline and Jim Geach for useful discussions. Facilities: XMM, JCMT, GMRT, VLA, Spitzer Published 2010 March 23|
|Subject Keywords:||galaxies: active; galaxies: evolution; galaxies: high-redshift; galaxies: starburst; infrared: galaxies; submillimeter: galaxies|
|Classification Code:||PACS: 98.54.Cm; 98.62.Ai; 95.85.Hp; 98.62.Bj; 98.54.Ep; 98.62.Py|
|Official Citation:||K. Coppin et al 2010 ApJ 713 503 doi: 10.1088/0004-637X/713/1/503|
|Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Jason Perez|
|Deposited On:||15 Apr 2010 20:46|
|Last Modified:||26 Dec 2012 11:58|
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