The Spitzer Spectroscopic Survey of the Small Magellanic Cloud (S4MC):
  Probing the Physical State of Polycyclic Aromatic Hydrocarbons in a
  Low-Metallicity Environment
  We present results of mid-infrared spectroscopic mapping observations of six
star-forming regions in the Small Magellanic Cloud from the Spitzer
Spectroscopic Survey of the SMC (S4MC). 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 micron features relative to the 11.3
micron feature and weak 8.6 and 17.0 micron 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 towards 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.
