Aromatic Features in M 101 HII Regions and Starburst Galaxies

The behavior of the aromatic features as a function of metallic-ity/radiation field hardness in M101 HII regions and starburst galaxies is investigated using Spitzer/IRS spectra. These spectra reveal that the 6.2, 7.8+8.6, and 11.3 µm aromatics have different dependencies on metallicity/radiation field hardness. Specifically, the 6.2 and 7.8+8.6 µm aromatics are weak or absent at a metallicity, 12+log(O/H), of ∼8.0 while the 11.3 µm feature remains relatively strong. These results apply to both the M101 HII regions and starbursts showing that HII regions can indeed be thought of as " mini-starbursts. " Comparison of this work with various candidate materials results in a good match with the annealing behavior in Quenched Carbonaceous Composite (QCC) which is an amorphous, hydrogenated, carbonaceous solid. The aromatic features are a family of dust emission features seen in many dusty astrophysical environments.1 µm and all the aromatics have been identified with C-H and CC bending and stretching modes of hydrocarbons containing aromatic rings (Tielens 2005). The shape and strength of the aro-matics have been seen to vary in single sources (Werner et al. 2004b), among various Galactic sources (van Diedenhoven et al. 2004), and among galaxies (Engelbracht et al. 2005; Madden et al. 2006; Wu et al. 2006). A number of different materials have been proposed as the carriers of these features since they were first discovered (Gillett et al. 1973) including Hydrogenated Amor-(Jones & d'Hendecourt 2000). The leading candidate material for the aromatics is PAH molecules and they are included in the modified " astronomical PAHs " form in dust grain models (Li & Draine 2001; Zubko et al. 2004). The preception that PAHs are the carrier of the aromatics can be seen by the common use of the term " PAH features " to refer to the aromatics. The identification of PAH molecules with the aromatics may be somewhat premature given that laboratory spectrum of a mix of PAHs does not match the observed wavelengths and strengths of the aromatics in detail (Hudgins & Allamandola 2004). As part of the Multiband Imaging Photometer for Spitzer (MIPS) Guaranteed Time Observations (GTO), two programs were carried out to explore the variations in the infrared (IR) Spectral Energy Distributions (SEDs) of massive star formation as a function of metallicity. The first program targets starburst galaxies with metallicities [12+log(O/H)] between 7.2 to 8.9, with emphasis on the lowest metallicity starbursts (Engelbracht et al. 2005). The 1