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Mid-infrared spectral diagnosis of submillimeter galaxies

Pope, A.; Chary, R.R.; Alexander, D.M.; Armus, L.; Dickinson, M.; Elbaz, D.; Frayer, D.; Scott, D.; Teplitz, H.

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A. Pope

R.R. Chary

L. Armus

M. Dickinson

D. Elbaz

D. Frayer

D. Scott

H. Teplitz


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.


Pope, A., Chary, R., Alexander, D., Armus, L., Dickinson, M., Elbaz, D., …Teplitz, H. (2008). Mid-infrared spectral diagnosis of submillimeter galaxies. Astrophysical Journal, 675(2), Article 1171.

Journal Article Type Article
Publication Date Mar 1, 2008
Deposit Date Jan 30, 2012
Publicly Available Date Jan 3, 2014
Journal Astrophysical Journal
Print ISSN 0004-637X
Electronic ISSN 1538-4357
Publisher American Astronomical Society
Peer Reviewed Peer Reviewed
Volume 675
Issue 2
Article Number 1171
Keywords Galaxies, Evolution, Starburst, Infrared, Submillimeter, Techniques, Spectroscopic.
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Published Journal Article (1.5 Mb)

Copyright Statement
© 2008. The American Astronomical Society. All rights reserved. Printed in the U.S.A.

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