2001A&A...365..571W

We present the results of a high angular resolution, multi-transition analysis of the molecular gas in M 82. The analysis is based on the two lowest transitions of ¹²CO and the ground transition of the rare isotopes ¹³CO and C¹⁸O 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. The calculations suggest that the kinetic temperature of the molecular gas is high in regions with strong star formation and drops towards the outer molecular lobes with less ongoing star formation. The location of the highest kinetic temperature is coincident with that of the mid infrared (MIR) peaks which trace emission from hot dust. The hot gas is associated with low H₂ densities while the cold gas in the outer molecular lobes has high H₂ densities. We find that CO intensities do not trace H₂ column densities well. Most of the molecular gas is distributed in a double-lobed distribution which surrounds the starburst. A detailed analysis of the conversion factor from CO intensity to H₂ column density shows that X_CO depends on the excitation conditions. We find X_CO∼T_kin^–1n(H₂)^1/2, as expected for virialized clouds.