The Abundance and Physical State of Polycyclic Aromatic Hydrocarbons in the Small Magellanic Cloud

Details The Abundance and Physical State of Polycyclic Aromatic Hydrocarbons in the Small Magellanic Cloud The Abundance and Physical State of Polycyclic Aromatic Hydrocarbons in the Small Magellanic Cloud

Jan 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010aas...21560635s&link_type=abstract

American Astronomical Society, AAS Meeting #215, #606.35; Bulletin of the American Astronomical Society, Vol. 36, p.1135

Astronomy and Astrophysics

Astronomy

Scientific paper

We present the results of two studies investigating the abundance and physical state of polycyclic aromatic hydrocarbons (PAHs) in the Small Magellanic Cloud (SMC). PAHs play important roles in the thermal balance and chemistry of the interstellar medium and their mid-IR emission bands are bright and detectable out to high redshift. 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 colors 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 to learn what processes set the abundance and physical state of PAHs. To study the PAH abundance, we model dust SEDs across the SMC using Spitzer Survey of the SMC (S3MC) photometry from 3.6 to 160 microns. We also use overlapping spectral mapping observations from 5 to 38 microns from the Spitzer Spectroscopic Survey of the Small Magellanic Cloud (S4MC) in selected regions to test the reliability of the PAH abundance as determined from SED fits using the Draine et al (2007) models. Our results show that the average PAH abundance in the SMC is low compared to the Milky Way (q_pah 0.6% vs. 4%) and variable--with high abundance (q_pah 1-2%) in molecular clouds and low abundance in the diffuse ISM. Based on these results, we suggest that PAHs are destroyed in the diffuse ISM of the SMC and/or PAHs are forming in molecular clouds. In addition, we use S4MC observations to diagnose the physical state of the PAHs from the ratios of their mid-IR bands. We find that PAHs in the SMC tend to be smaller and less ionized than in higher metallicity galaxies.

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