The spectral energy distribution of CO lines in M 82

We present an analysis of the excitation conditions of the molecular gas in the streamers and the outflow of M82 based on observations obtained at the IRAM 30m telescope. Our analysis of J = 1 -> 0 and J = 2 -> 1 transitions of CO and (CO)-C-13 and the CO( J = 3 -> 2) line in 13 regions outside the central starburst disk shows that the gas density within the streamer/outflow system is about an order of magnitude lower (log(n(H2)) approximate to 3.0 cm(-3)) than in the central molecular disk. We have used an LVG model and data from the literature to constrain the flux density in each CO transition ( the CO line SED) arising from the streamer/outflow system and the central starburst disk itself. Globally, we find that the CO flux density up to the J = 3 -> 2 line is dominated by the di. use outer regions while lines above the J = 5 -> 4 transition are almost exclusively emitted by the central starburst disk. We compare the CO line SED of M82 to CO observations of galaxies at high redshift and suggest that small high-J/low-J CO flux density ratios ( observed in some of these sources) are not necessarily caused by a different excitation of the central molecular gas concentration, but may result from an additional, more extended and di. use gas reservoir around these systems, reminiscent of the situation in M82.