The effect of violent star formation on the state of the molecular gas in M 82

We present the results of a high angular resolution, multi-transition analy sis of the molecular gas in M82. The analysis is based on the two lowest tr ansitions of (CO)-C-12 and the ground transition of the rare isotopes (CO)- C-13 and (CO)-O-18 measured with the PdBI, the BIMA array and the IRAM 30 m telescope. In order to address the question of how the intrinsic molecular cloud properties are influenced by massive star formation we have carried out radiative transfer calculations based on the observed CO line ratios. T he calculations suggest that the kinetic temperature of the molecular gas i s high in regions with strong star formation and drops towards the outer mo lecular lobes with less ongoing star formation. The location of the highest kinetic temperature is coincident with that of the mid infrared (MTR) peak s which trace emission from hot dust. The hot gas is associated with low Hz densities while the cold gas in the outer molecular lobes has high Ha dens ities. We find that CO intensities do not trace Ha column densities well. M ost of the molecular gar; is distributed in a double-lobed distribution whi ch surrounds the starburst. A detailed analysis of the conversion factor fr om CO intensity to Ha column density shows that X-CO depends on the excitat ion conditions. We find X-CO similar to T-kin(-1) n(H-2)(1/2), as expected for virialized clouds.