Mid-infrared Spectroscopy of Candidate Agn-dominated Submillimeter Galaxies

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 (S8μm/S4.5μm > 2; i.e. likely power-law mid-infrared SEDs). 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 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 < 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 AGN dominate the bolometric emission of the SMG when the results are extrapolated to longer infrared wavelengths, implying that AGN 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 S8μm/S4.5μm > 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. Subject headings: galaxies: evolution — galaxies: high-redshift – galaxies: starburst — galaxies: active — infrared: galaxies — submillimeter 1 Institute for Computational Cosmology, Durham University, South Road, Durham, DH1 3LE, UK 2 National Optical Astronomy Observatory, 950 N. Cherry Ave., Tucson, AZ, 85719 3 Spitzer Fellow 4 NSF Astronomy and Astrophysics Postdoctoral Fellow 5 The Observatories of the Carnegie Institution for Science, 813 Santa Barbara St., Pasadena, CA, 91101, USA 6 Department of Physics, Durham University, South Road, Durham, DH1 3LE, UK 7 Scottish Universities Physics Alliance (SUPA), Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh, EH9 3HJ, UK 8 Steward Observatory, University of Arizona, 933 N. Cherry Ave., Tuscon, AZ, 85721 9 UK ATC, Science and Technology Facilities Council, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK 10 Center for Astrophysics and Space Astronomy, University of Colorado, Boulder, CO 80309 11 Caltech, 249-17, Pasadena, CA 91125, USA 12 Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK 13 Astrophysics Group, Blackett Laboratory, Imperial College, Prince Consort Rd., London SW7 2BW, UK 14 The School of Physics and Astronomy, University of Nottingham, University Park, Nottingham NG7 2RD, UK 15 School of Physics and Astronomy, Cardiff University, 5, The Parade, Cardiff CF24 3YB, UK 16 Astronomy Centre, University of Sussex, Falmer, Brighton BN1 9QH, UK 17 Instituto Nacional de Astrof́ısica, Óptica y Electrónica, Apartado Postal 51 y 216, 72000 Puebla, Pue., Mexico 18 Mullard Space Science Laboratory (MSSL), University College London, Holmbury St. Mary, Dorking, Surrey RH5 6NT, UK