2012A&A...541A..10Y

We investigate the properties of hydrocarbon grains in the galactic superwind of M 82. With AKARI, we performed near-infrared (2.5-4.5µm) spectroscopic observations of 34 regions in M 82 including its northern and southern halos. Many of the spectra show strong emission at 3.3µm caused by polycyclic aromatic hydrocarbons (PAHs) and relatively weak features at 3.4-3.6µm caused by aliphatic hydrocarbons. In particular, we clearly detect the PAH 3.3µm emission and the 3.4-3.6µm 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 3.4-3.6µm features to the 3.3µm feature intensity significantly increase with distance from the galactic center, while the ratios of the 3.3µm feature to the AKARI 7µm band intensity do not. 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 3.4-3.6µm features are unusually abundant in the halo, suggesting that small carbonaceous grains are produced by shattering of larger grains in the galactic superwind.