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

TL;DR

This study uses AKARI near-infrared spectroscopy to analyze hydrocarbon emissions in M 82's galactic superwind, revealing abundant small PAHs and aliphatic hydrocarbons even in the galaxy's halo, indicating grain shattering processes.

Contribution

First detailed near-infrared spectroscopic analysis of hydrocarbon features in the halo of M 82, showing the presence and variation of PAHs and aliphatic hydrocarbons at large distances from the galactic center.

Findings

Detection of strong PAH 3.3 um emission in halo regions

Increase in aliphatic feature ratios with distance from center

Presence of small hydrocarbons in harsh halo environment

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 um) 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 um due to polycyclic aromatic hydrocarbons (PAHs) and relatively weak features at 3.4 - 3.6 um due to aliphatic hydrocarbons. In particular, we clearly detect the PAH 3.3 um emission and the 3.4 - 3.6 um 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 um features to the 3.3 um feature intensity significantly increase with distance from the galactic center, while the ratios of the 3.3 um feature to the AKARI 7 um band intensity do not. Conclusions. Our results clearly confirm the presence of small PAHs even in a harsh environment of the halo of M 82. The results also reveal that the aliphatic hydrocarbons emitting the 3.4 - 3.6 um features are unusually abundant in the halo, suggesting that small carbonaceous grains are produced by shattering of larger grains in the galactic superwind.