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The Spitzer Surveys of the Small Magellanic Cloud: Insightsinto the Life-Cycle of Polycyclic Aromatic Hydrocarbons

Published online by Cambridge University Press:  30 March 2011

C. Joblin ,
A.G.G.M. Tielens ,
K.M. Sandstrom ,
A.D. Bolatto ,
B.T. Draine ,
C. Bot  and
S. Stanimirovic
K.M. Sandstrom
Affiliation:
Max-Planck-Institute for Astronomy, Konigstuhl 17, 69117 Heidelberg, Germany
A.D. Bolatto
Affiliation:
Department of Astronomy and Laboratory for Millimeter-Wave Astronomy, University of Maryland, College Park, MD, USA
B.T. Draine
Affiliation:
Department of Astrophysical Sciences, Princeton University, NJ, USA
C. Bot
Affiliation:
Observatoire Astronomiques de Strasbourg, Université Louis Pasteur, 67000 Strasbourg, France
S. Stanimirovic
Affiliation:
Department of Astronomy, University of Wisconsin, Madison, WI, USA

Abstract

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We present the results of two studies investigating the abundance and physical state ofpolycyclic aromatic hydrocarbons (PAHs) in the Small Magellanic Cloud (SMC). Observationswith ISO and Spitzer have shown that PAHs are deficient in low-metallicity galaxies. Inparticular,galaxies with 12 + log(O/H)  <  8 show mid-IR SEDs and spectraconsistent with low PAH abundance. The SMC provides a unique opportunity to map the PAHemission in a low-metallicity (12  +  log(O/H)  ~ 8) galaxy at high spatialresolution and sensitivity to learn about the PAH life cycle. Using mid- and far-IRphotometry from the Spitzer Survey of the SMC (S3MC) and mid-IR spectralmapping from the Spitzer Spectroscopic Survey of the SMC (S4MC) we determinethe PAH abundance across the galaxy. We find that the SMC PAH abundance is low compared tothe Milky Way and variable,with high abundance in molecular regions and low abundance inthe diffuse ISM. From the variations of the mid-IR band strengths,we show that PAHs inthe SMC are smaller and more neutral than their counterparts in more metal-rich galaxies.Based on the results of these two studies we propose that PAHs in the SMC are formed witha size distribution shifted towards smaller grains and are therefore easier to destroyunder typical diffuse ISM conditions. The distribution of PAH abundance in the SMCsuggests that PAH formation in molecular clouds is an important process. We discuss theimplications of these results for our understanding of the PAH life-cycle both atlow-metallicity and in the Milky Way.

Type
Research Article
Information
European Astronomical Society Publications Series , Volume 46: PAHs and the Universe: A Symposium to Celebrate the 25thAnniversary of the PAH Hypothesis , 2011 , pp. 215 - 221
Copyright
© EAS, EDP Sciences 2011

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