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

¹ Max-Planck-Institute for Astronomy, Konigstuhl 17, 69117 Heidelberg, Germany
² Department of Astronomy and Laboratory for Millimeter-Wave Astronomy, University of Maryland, College Park, MD, USA
³ Department of Astrophysical Sciences, Princeton University, NJ, USA
⁴ Observatoire Astronomiques de Strasbourg, Université Louis Pasteur, 67000 Strasbourg, France
⁵ Department of Astronomy, University of Wisconsin, Madison, WI, USA

Abstract

We present the results of two studies investigating the abundance and physical state of polycyclic aromatic hydrocarbons (PAHs) in the Small Magellanic Cloud (SMC). Observations with ISO and Spitzer have shown that PAHs are deficient in low-metallicity galaxies. In particular, galaxies with 12 + log(O/H)  <  8 show mid-IR SEDs and spectra consistent with low PAH abundance. The SMC provides a unique opportunity to map the PAH emission in a low-metallicity (12  +  log(O/H)  ~ 8) galaxy at high spatial resolution and sensitivity to learn about the PAH life cycle. Using mid- and far-IR photometry from the Spitzer Survey of the SMC (S³MC) and mid-IR spectral mapping from the Spitzer Spectroscopic Survey of the SMC (S⁴MC) we determine the PAH abundance across the galaxy. We find that the SMC PAH abundance is low compared to the Milky Way and variable, with high abundance in molecular regions and low abundance in the diffuse ISM. From the variations of the mid-IR band strengths, we show that PAHs in the 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 with a size distribution shifted towards smaller grains and are therefore easier to destroy under typical diffuse ISM conditions. The distribution of PAH abundance in the SMC suggests that PAH formation in molecular clouds is an important process. We discuss the implications of these results for our understanding of the PAH life-cycle both at low-metallicity and in the Milky Way.