2006A&A...450L..13M

The nuclear starburst region in M 82 shows systematical low abundances of some complex molecules when compared with other starburst galaxies. This is likely related to a presumably photodissociation dominated environment. In particular, methanol is known to show relatively low abundance because it is easily photodissociated. We present a multilevel study of the emission of methanol, detected for the first time in this galaxy, and discuss the origin of its emission. Observations of three transitions of CH₃OH towards the center and two positions around the nucleus of M 82 are presented. Two different components are found, one with high excitation (n(H₂)∼10⁶cm^–3, T_rot∼20K) and the other with low excitation (n(H₂)∼10⁴cm^–3, T_rot∼5K). The high observed methanol abundance of a few 10^–9 can only be explained if injection of methanol from dust grains is taken into account. While the overall [CH₃OH]/[NH₃] ratio is much larger than observed towards other starbursts, the dense high excitation component shows a similar value to that found in NGC253 and Maffei2. Our observations suggest the molecular material in M 82 to be formed by dense warm cores, shielded from the UV radiation and similar to the molecular clouds in other starbursts, surrounded by a less dense photodissociated halo. The dense warm cores are likely the location of recent and future star formation within M 82.