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The Mid-Infrared Spectrum of Star-forming Galaxies: Global Properties of Polycyclic Aromatic Hydrocarbon Emission

Smith, J. D. T. and Draine, B. T. and Dale, D. A. and Moustakas, J. and Kennicutt, R. C., Jr. and Helou, G. and Armus, L. and Roussel, H. and Sheth, K. and Bendo, G. J. and Buckalew, B. A. and Calzetti, D. and Engelbracht, C. W. and Gordon, K. D. and Hollenbach, D. J. and Li, A. and Malhorta, S. and Murphy, E. J. and Walter, F. (2007) The Mid-Infrared Spectrum of Star-forming Galaxies: Global Properties of Polycyclic Aromatic Hydrocarbon Emission. Astrophysical Journal, 656 (2). pp. 770-791. ISSN 0004-637X. doi:10.1086/510549.

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We present a sample of low-resolution 5-38 μm Spitzer IRS spectra of the inner few square kiloparsecs of 59 nearby galaxies spanning a large range of star formation properties. A robust method for decomposing mid-infrared galaxy spectra is described and used to explore the behavior of PAH emission and the prevalence of silicate dust extinction. Evidence for silicate extinction is found in ~1/8 of the sample, at strengths that indicate that most normal galaxies undergo A_V ≲ 3 mag averaged over their centers. The contribution of PAH emission to the total infrared power is found to peak near 10% and extend up to ~20% and is suppressed at metallicities Z ≲ Z_☉/4, as well as in low-luminosity AGN environments. Strong interband PAH feature strength variations (2-5 times) are observed, with the presence of a weak AGN and, to a lesser degree, increasing metallicity shifting power to the longer wavelength bands. A peculiar PAH emission spectrum with markedly diminished 5-8 μm features arises among the sample solely in systems with relatively hard radiation fields harboring low-luminosity AGNs. The AGNs may modify the emitting grain distribution and provide the direct excitation source of the unusual PAH emission, which cautions against using absolute PAH strength to estimate star formation rates in systems harboring active nuclei. Alternatively, the low star formation intensity often associated with weak AGNs may affect the spectrum. The effect of variations in the mid-infrared spectrum on broadband infrared surveys is modeled and points to more than a factor of 2 uncertainty in results that assume a fixed PAH emission spectrum, for redshifts z = 0-2.5.

Item Type:Article
Related URLs:
URLURL TypeDescription
Smith, J. D. T.0000-0003-1545-5078
Draine, B. T.0000-0002-0846-936X
Dale, D. A.0000-0002-5782-9093
Kennicutt, R. C., Jr.0000-0001-5448-1821
Helou, G.0000-0003-3367-3415
Armus, L.0000-0003-3498-2973
Sheth, K.0000-0002-5496-4118
Calzetti, D.0000-0002-5189-8004
Murphy, E. J.0000-0001-7089-7325
Walter, F.0000-0003-4793-7880
Additional Information:© 2007 American Astronomical Society. Print publication: Issue 2 (2007 February 20); received 2006 May 5; accepted for publication 2006 October 29. The authors thank H. Spoon for assistance with spline-based techniques, K. Masters for early access to her updated local flow model, C. Markwardt for his indispensable IDL L-M fitting package, J. Chiar, E. Peeters, C. Papovich,D. Elbaz, and R. Chary for helpful discussions, and an anonymous referee for many useful suggestions. This work made use of the NASA/ IPAC Extragalactic Database (NED), operated by JPL/Caltech, under contract with NASA. Support for this work, part of the Spitzer Space Telescope Legacy Science Program, was provided by NASA through contract 1224769 issued by JPL/Caltech under contract 1407. B. T. D. has been partially supported by NSF grant AST 04-06883.
Funding AgencyGrant Number
NSFAST 04-06883
Subject Keywords:galaxies: active; galaxies: ISM; infrared: galaxies; techniques: spectroscopic
Issue or Number:2
Record Number:CaltechAUTHORS:20100211-104124267
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Official Citation:The Mid-Infrared Spectrum of Star-forming Galaxies: Global Properties of Polycyclic Aromatic Hydrocarbon Emission J. D. T. Smith, B. T. Draine, D. A. Dale, J. Moustakas, R. C. Kennicutt, Jr., G. Helou, L. Armus, H. Roussel, K. Sheth, G. J. Bendo, B. A. Buckalew, D. Calzetti, C. W. Engelbracht, K. D. Gordon, D. J. Hollenbach, A. Li, S. Malhotra, E. J. Murphy, and F. Walter 2007 ApJ 656 770-791 doi: 10.1086/510549
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:17453
Deposited By: Jason Perez
Deposited On:11 Feb 2010 21:13
Last Modified:08 Nov 2021 23:35

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