Letter to the Editor

Methanol detection in M 82

¹ Instituto de Radioastronomía Milimétrica, Avda. Divina Pastora 7, Local 20, 18012 Granada, Spain e-mail: martin@iram.es
² Departamento de Astrofísica Molecular e Infrarroja, Instituto de Estructura de la Materia, CSIC, Serrano 121, 28006 Madrid, Spain

Received: 14 December 2005
Accepted: 7 March 2006

Abstract

Context.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.Aims.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.Methods.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 (, K) and the other with low excitation (, K).Results.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 NGC 253 and Maffei 2. Conclusions.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.