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Author(s): Crocker, Alison F; Calzetti, Daniela; Thilker, David A; Aniano, Gonzalo; Draine, Bruce T.; et al

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Abstract: Combining H alpha and IRAC images of the nearby spiral galaxy NGC 628, we find that between 30% and 43% of its 8 mu m dust emission is not related to recent star formation. Contributions from dust heated by young stars are separated by identifying H II regions in the H alpha map and using these areas as a mask to determine the 8 mu m dust emission that must be due to heating by older stars. Corrections are made for sub-detection-threshold H II regions, photons escaping from H II regions, and for young stars not directly associated with H II regions (i.e., 10-100 Myr old stars). A simple model confirms that this amount of 8 mu m emission can be expected given dust and PAH absorption cross sections, a realistic star formation history, and the observed optical extinction values. A Fourier power spectrum analysis indicates that the 8 mu m dust emission is more diffuse than the II alpha emission (and similar to observed H I), supporting our analysis that much of the 8 mu m-emitting dust is heated by older stars. The 8 mu m dust-to-H alpha emission ratio declines with galactocentric radius both within and outside of H II regions, probably due to a radial increase in disk transparency. In the course of this work, we have also found that intrinsic diffuse H alpha fractions may be lower than previously thought in galaxies, if the differential extinction between H II regions and diffuse regions is taken into account.
Publication Date: 10-Jan-2013
Electronic Publication Date: 18-Dec-2012
Citation: Crocker, Alison F, Calzetti, Daniela, Thilker, David A, Aniano, Gonzalo, Draine, Bruce T, Hunt, Leslie K, Kennicutt, Robert C, Sandstrom, Karin, Smith, JDT. (2013). QUANTIFYING NON-STAR-FORMATION-ASSOCIATED 8 mu m DUST EMISSION IN NGC 628. ASTROPHYSICAL JOURNAL, 762 (10.1088/0004-637X/762/2/79
DOI: doi:10.1088/0004-637X/762/2/79
ISSN: 0004-637X
EISSN: 1538-4357
Type of Material: Journal Article
Journal/Proceeding Title: ASTROPHYSICAL JOURNAL
Version: Final published version. Article is made available in OAR by the publisher's permission or policy.
Notes: Supplementary data may be available at and

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