Properties of UIR bands in NGC 6946 based on mid-infrared imaging and spectroscopy with Infrared Camera on board AKARI

We present results on mid-infrared imaging and spectroscopic observations of the nearby late-type spiral NGC 6946 with the Infrared Camera on board AKARI. Based on mid-infrared imaging with the S7 (7 mu m) and S11(11 mu m) bands, we found that the S7/S11 ratios have larger values in the arm region, containing many star-forming regions, than in the interarm. Slit spectra of an interarm and a star-forming region show a series of unidentified infrared (UIR) bands at 6.2, 7.7, 8.6, and 11.2 mu m. The strengths of the 6.2, 7.7, and 8.6 mu m bands are larger relative to the 11.2 mu m band in the star-forming region, than in the interarm, even if the interstellar extinction of A(V) = 3 mag towards the star-forming region is taken into account. The increase in the 6.2 mu m and 7.7 mu m features relative to the 11.2 mu m feature is consistent with the ionization model of PAHs. The ratio of the UIR features to the plateau emission under the 7.7 mu m and 8.6 mu m features and/or the ratio of the 7.6 mu m/7.8 mu m components in the 7.7 mu m feature increase in the star-forming region compared to the interarm. This variation can be accounted for by the hypothesis that the photo-evaporation of PAH clusters produce small free-flying PAHs in the star-forming region. The ratios of 6.2 mu m/11.2 mu m, 7.7 mu m/11.2 mu m, and 8.6 mu m/11.2 mu m may be tools to measure the star-formation activity in remote galaxies if these ratios and their variations are well examined and established for a large sample.