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

Abstract

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 μm) and S11(11 μm) 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 μm. The strengths of the 6.2, 7.7, and 8.6 μm bands are larger relative to the 11.2 μm band in the star-forming region, than in the interarm, even if the interstellar extinction of AV = 3 mag towards the star-forming region is taken into account. The increase in the 6.2 μm and 7.7 μm features relative to the 11.2 μm feature is consistent with the ionization model of PAHs. The ratio of the UIR features to the plateau emission under the 7.7 μm and 8.6 μm features and/or the ratio of the 7.6 μm/7.8 μm components in the 7.7 μm 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 μm/11.2 μm, 7.7 μm/11.2 μm, and 8.6 μm/11.2 μm 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. © 2007. Astronomical Society of Japan.