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 \to 0 and J =2 \to 1 transitions of CO and ^ { 13 } CO and the CO ( J =3 \to 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 _ { { { H } _ { 2 } } } ) \approx 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 \to 2 line is dominated by the diffuse outer regions while lines above the J =5 \to 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 .