A lambda = 1.3 mm and 2 mm molecular line survey towards M82
  We study the chemical complexity towards the central parts of the starburst
galaxy M82, and investigate the role of certain molecules as tracers of the
physical processes in the galaxy circumnuclear region. We carried out a
spectral line survey with the IRAM-30m telescope towards the northeastern
molecular lobe of M82. 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. 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 calculated 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 M82 and NGC253. This
comparison enables us to establish the chemical differences between the
products of the strong photon-dominated regions (PDRs) driving the heating in
M82, and the large-scale shocks that influence the properties of the molecular
clouds in the nucleus of NGC253. 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 M82 than in NGC253, 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 NGC253 than
in M82.
