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 M 82 based on observations obtained at the IRAM 30 m telescope. Our analysis of J=1→0 and J=2→1 transitions of CO and 13 CO 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(nH 2) ≈ 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 diffuse 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 M 82 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 diffuse gas reservoir around these systems, reminiscent of the situation in M 82.