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

¹ Graduate School of Science, Nagoya University, Furo-cho, Chikusa-ku, 464-8602 Nagoya Japan
e-mail: yamagishi@u.phys.nagoya-u.ac.jp
² Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, 113-0033 Tokyo, Japan
³ Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 3-1-1 Yoshinodai, Chuo-ku, 252-5210 Sagamihara, Japan
⁴ Korea Astronomy and Space Science Institute, 776 Daedeok-daero, Yuseong, 305-348 Daejeon, Korea

Received: 30 January 2012
Accepted: 12 March 2012

Abstract

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 μm) 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 μm caused by polycyclic aromatic hydrocarbons (PAHs) and relatively weak features at 3.4–3.6 μm caused by aliphatic hydrocarbons. In particular, we clearly detect the PAH 3.3 μm emission and the 3.4–3.6 μm 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 μm features to the 3.3 μm feature intensity significantly increase with distance from the galactic center, while the ratios of the 3.3 μm feature to the AKARI 7 μm 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 μm features are unusually abundant in the halo, suggesting that small carbonaceous grains are produced by shattering of larger grains in the galactic superwind.