AKARI near-infrared spectroscopy of the aromatic and aliphatic hydrocarbon emission features in the galactic superwind of M 82

Aims. We investigate the properties of hydrocarbon grains in the galactic superwind of M 82. Methods. With AKARI, we performed near-infrared (2.5-4.5 mu m) spectroscopic observations of 34 regions in M 82 including its northern and southern halos. Results. Many of the spectra show strong emission at 3.3 mu m caused by polycyclic aromatic hydrocarbons (PAHs) and relatively weak features at 3.4-3.6 mu m caused by aliphatic hydrocarbons. In particular, we clearly detect the PAH 3.3 mu m emission and the 3.4-3.6 mu m features in halo regions, which are located at a distance of 2 kpc away from the galactic center. We find that the ratios of the 3.4-3.6 mu m features to the 3.3 mu m feature intensity significantly increase with distance from the galactic center, while the ratios of the 3.3 mu m feature to the AKARI 7 mu m band intensity do not. Conclusions. Our results clearly confirm the presence of small PAHs even in the harsh environment of the halo of M 82. The results also reveal that the aliphatic hydrocarbons emitting the 3.4-3.6 mu m features are unusually abundant in the halo, suggesting that small carbonaceous grains are produced by shattering of larger grains in the galactic superwind.