Mid-infrared spectroscopy of candidate active galactic nuclei-dominated submillimeter galaxies

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Title: Mid-infrared spectroscopy of candidate active galactic nuclei-dominated submillimeter galaxies
Authors: Coppin, K
Pope, A
Menendez-Delmestre, K
Alexander, DM
Dunlop, JS
Egami, E
Gabor, J
Ibar, E
Ivison, RJ
Austermann, JE
Blain, AW
Chapman, SC
Clements, DL
Dunne, L
Dye, S
Farrah, D
Hughes, DH
Mortier, AMJ
Page, MJ
Rowan-Robinson, M
Scott, D
Simpson, C
Smail, I
Swinbank, AM
Vaccari, M
Yun, MS
Item Type: Journal Article
Abstract: Spitzer spectroscopy has revealed that sime80% of submm galaxies (SMGs) are starburst (SB)-dominated in the mid-infrared. Here we focus on the remaining sime20% 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 (gsim50%) 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.
Issue Date: 10-Apr-2010
Date of Acceptance: 28-Feb-2010
URI: http://hdl.handle.net/10044/1/60480
DOI: https://dx.doi.org/10.1088/0004-637X/713/1/503
ISSN: 0004-637X
Publisher: American Astronomical Society
Start Page: 503
End Page: 519
Journal / Book Title: Astrophysical Journal
Volume: 713
Issue: 1
Copyright Statement: ©2010. The American Astronomical Society. All rights reserved.
Sponsor/Funder: Science and Technology Facilities Council (STFC)
Science and Technology Facilities Council (STFC)
Science and Technology Facilities Council (STFC)
Funder's Grant Number: ST/G003874/1
Keywords: Science & Technology
Physical Sciences
Astronomy & Astrophysics
galaxies: active
galaxies: evolution
galaxies: high-redshift
galaxies: starburst
infrared: galaxies
submillimeter: galaxies
0201 Astronomical And Space Sciences
0305 Organic Chemistry
0306 Physical Chemistry (Incl. Structural)
Publication Status: Published
Online Publication Date: 2010-03-23
Appears in Collections:Physics
Faculty of Natural Sciences

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