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Published January 1, 2012 | Published
Journal Article Open

The Spitzer Spectroscopic Survey of the Small Magellanic Cloud (S4MC): Probing the Physical State of Polycyclic Aromatic Hydrocarbons in a Low-metallicity Environment

Abstract

We present results of mid-infrared spectroscopic mapping observations of six star-forming regions in the Small Magellanic Cloud (SMC) from the Spitzer Spectroscopic Survey of the SMC (S^4MC). We detect the mid-IR emission from polycyclic aromatic hydrocarbons (PAHs) in all of the mapped regions, greatly increasing the range of environments where PAHs have been spectroscopically detected in the SMC. We investigate the variations of the mid-IR bands in each region and compare our results to studies of the PAH bands in the SINGS sample and in a sample of low-metallicity starburst galaxies. PAH emission in the SMC is characterized by low ratios of the 6-9 μm features relative to the 11.3 μm feature and weak 8.6 and 17.0 μm features. Interpreting these band ratios in the light of laboratory and theoretical studies, we find that PAHs in the SMC tend to be smaller and less ionized than those in higher metallicity galaxies. Based on studies of PAH destruction, we argue that a size distribution shifted toward smaller PAHs cannot be the result of processing in the interstellar medium, but instead reflects differences in the formation of PAHs at low metallicity. Finally, we discuss the implications of our observations for our understanding of the PAH life-cycle in low-metallicity galaxies—namely that the observed deficit of PAHs may be a consequence of PAHs forming with smaller average sizes and therefore being more susceptible to destruction under typical interstellar medium conditions.

Additional Information

© 2012 The American Astronomical Society. Received 2011 July 15; accepted 2011 September 12; published 2011 December 8. We thank Chad Engelbracht and Karl Gordon for providing the starburst sample PAHFIT results, Erik Muller for providing the SMC Hi map, and Carl Starkey and Remy Indebetouw for their help with the "dark settle" artifact correction. We thank the anonymous referee for helpful comments on the structure of the paper. This work is based on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. This research was supported in part by NASA through awards issued by JPL/Caltech (NASA-JPL Spitzer grant 1264151 awarded to Cycle 1 project 3316, and grants 1287693 and 1289519 awarded to Cycle 3 project 30491). A.D.B. wishes to acknowledge travel support from FONDECYT(CHILE) grant No. 1080335. A.D.B. wishes to acknowledge partial support from grants NSF AST- 0838178 and NSF AST-0955836, as well as a Cottrell Scholar award from the Research Corporation for Science Advancement RCSA 19968. A.K.L. is supported in part by an NSF grant AST-1109039. M.R. wishes to acknowledge support from FONDECYT(CHILE) grant No. 1080335 and is supported by the Chilean Center for Astrophysics FONDAP No. 15010003. This research has made use of NASA's Astrophysics Data System. Facility: Spitzer ()

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August 22, 2023
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