Mid-infrared spectroscopy of candidate AGN-dominated submillimeter galaxies

Kristen Coppin, A. Pope, K. Menendez-Delmestre, D. M. Alexander, J. S. Dunlop, E. Egami, J. Gabor, Edo Ibar, R. J. Ivison, J. E. Austermann, A. W. Blain, S. C. Chapman, D. L. Clements, L. Dunne, S. Dye, D. Farrah, D. H. Hughes, A. M. J. Mortier, M. J. Page, M. Rowan-RobinsonD. Scott, C. Simpson, Ian Smail, A. M. Swinbank, M. Vaccari, M. S. Yun

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Abstract

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 (AGN). We have obtained Spitzer-IRS spectroscopy of a sample of eight SMGs which are candidates for harboring powerful AGN on the basis of IRAC color-selection (S8/S4.5>2; i.e. likely power-law mid-infrared SEDs). SMGs with an AGN dominating (>50%) their mid-infrared emission could represent `missing link' sources in an evolutionary sequence involving a major merger. First of all, we detect PAH features in all of the SMGs, indicating redshifts from 2.5-3.4, demonstrating the power of the mid-infrared to determine redshifts for these optically faint dusty galaxies. Secondly, 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 1.65 works well at selecting mid-infrared energetically dominant AGN 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.
Original languageEnglish
Pages (from-to)503-519
JournalThe Astrophysical Journal
Volume713
DOIs
Publication statusPublished - 2010

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