A λ = 1.3 mm and 2 mm molecular line survey towards M 82 ⋆
Instituto de Radioastronomía Milimétrica (IRAM), Avda. Divina Pastora 7, Local 20, 18012 Granada, Spain
2 Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK
3 European Southern Observatory, Avda. Alonso de Córdova 3107, Vitacura, Casilla 19001, Santiago 19, Chile
4 Centro de Astrobiología (CSIC-INTA), Ctra. de Torrejón Ajalvir km 4, 28850 Torrejón de Ardoz, Madrid, Spain
5 Joint ALMA Observatory, Avda. Alonso de Córdova 3107, Vitacura, Santiago, Chile
6 Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
7 Astronomy Department, King Abdulaziz Univeristy, PO Box 80203, 21589 Jeddah, Saudi Arabia
8 National Centre for Radio Astrophysics, TIFR, Ganeshkhind, Pune 411007, India
Received: 2 June 2011
Accepted: 8 September 2011
Aims. We study the chemical complexity towards the central parts of the starburst galaxy M 82, and investigate the role of certain molecules as tracers of the physical processes in the galaxy circumnuclear region.
Methods. We carried out a spectral line survey with the IRAM-30 m telescope towards the northeastern molecular lobe of M 82. It covers the frequency range between 129.8 GHz and 175.0 GHz in the 2 mm atmospheric window, and between 241.0 GHz and 260.0 GHz in the 1.3 mm atmospheric window.
Results. Sixty-nine spectral features corresponding to 18 different molecular species are identified. In addition, three hydrogen recombination lines are detected. The species NO, H2S, H2CS, NH2CN, and CH3CN are detected for the first time in this galaxy. Assuming local thermodynamic equilibrium, we determine the column densities of all the detected molecules. We also calculate upper limits to the column densities of fourteen other important, but undetected, molecules, such as SiO, HNCO, or OCS. We compare the chemical composition of the two starburst galaxies M 82 and NGC 253. This comparison enables us to establish the chemical differences between the products of the strong photon-dominated regions driving the heating in M 82, and the large-scale shocks that influence the properties of the molecular clouds in the nucleus of NGC 253.
Conclusions. Overall, both sources have different chemical compositions. Some key molecules highlight the different physical processes dominating both central regions. Examples include CH3CCH, c-C3H2, or CO+, the abundances of which are clearly higher in M 82 than in NGC 253, pointing at photodissociating regions. On the other hand, species such as CH2NH, NS, SiO, and HOCO+ have abundances of up to one order of magnitude higher in NGC 253 than in M 82.
Key words: ISM: molecules / galaxies: nuclei / galaxies: abundances / radio lines: ISM / galaxies: starburst / galaxies: individual: M 82
Appendix is available in electronic form at http://www.aanda.org
© ESO, 2011