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Mid-Infrared Galaxy Classification Based on Silicate Obscuration and PAH Equivalent Width

Spoon, H. W. W. and Marshall, J. A. and Houck, J. R. and Elitzur, M. and Hao, L. and Armus, L. and Brandl, B. R. and Charmandaris, V. (2007) Mid-Infrared Galaxy Classification Based on Silicate Obscuration and PAH Equivalent Width. Astrophysical Journal Letters, 654 (1). L49-L52. ISSN 2041-8205. doi:10.1086/511268.

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We present a new diagnostic diagram for mid-infrared spectra of infrared galaxies based on the equivalent width of the 6.2 μm PAH emission feature and the strength of the 9.7 μm silicate feature. Based on the positions in this diagram, we classify galaxies into nine classes ranging from continuum-dominated AGN hot dust spectra and PAH-dominated starburst spectra to absorption-dominated spectra of deeply obscured galactic nuclei. We find that galaxies are systematically distributed along two distinct branches: one of AGN and starburst-dominated spectra and one of deeply obscured nuclei and starburst-dominated spectra. The separation into two branches likely reflects a fundamental difference in the dust geometry in the two sets of sources: clumpy versus nonclumpy obscuration. Spectra of ULIRGs are found along the full length of both branches, reflecting the diverse nature of the ULIRG family.

Item Type:Article
Related URLs:
URLURL TypeDescription
Spoon, H. W. W.0000-0002-8712-369X
Marshall, J. A.0000-0001-7712-8465
Elitzur, M.0000-0001-8143-3550
Armus, L.0000-0003-3498-2973
Charmandaris, V.0000-0002-2688-1956
Additional Information:© 2007 American Astronomical Society. Print publication: Issue 1 (2007 January 1); received 2006 October 26; accepted for publication 2006 November 14; published 2006 December 18. The authors wish to thank Jero´nimo Bernard-Salas, Nancy Levenson, He´le`ne Roussel, Matthew Sirocky, Alexander Tielens, and Dan Weedman for discussions, and Vandana Desai for help with the data reduction. Support for this work was provided by NASA through contract 1257184 issued by the Jet Propulsion Laboratory, California Institute of Technology under NASA contract 1407. H.W.W. S. was supported under this contract through the Spitzer Space Telescope Fellowship Program.
Group:Infrared Processing and Analysis Center (IPAC)
Funding AgencyGrant Number
NASA Spitzer FellowshipUNSPECIFIED
Subject Keywords:galaxies: active; galaxies: ISM; galaxies: starburst; infrared: galaxies
Issue or Number:1
Record Number:CaltechAUTHORS:20100205-112519472
Persistent URL:
Official Citation:Mid-Infrared Galaxy Classification Based on Silicate Obscuration and PAH Equivalent Width H. W. W. Spoon, J. A. Marshall, J. R. Houck, M. Elitzur, L. Hao, L. Armus, B. R. Brandl, and V. Charmandaris 2007 ApJ 654 L49-L52 doi: 10.1086/511268
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:17401
Deposited By: Jason Perez
Deposited On:10 Feb 2010 22:33
Last Modified:08 Nov 2021 23:35

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