Properties of UIR Bands in NGC6946 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 NGC6946 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/S11ratios 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.