The Effect of Violent Star Formation on the State of the Molecular Gas in M 82

Abstract

Abstract. 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 12 CO and the ground transition of the rare isotopes 13 CO and C 18 O measured with the PdBI, the BIMA array and the IRAM 30m 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 H 2 densities while the cold gas in the outer molecular lobes has high H 2 densities. We find that CO intensities do not trace H 2 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 H 2 column density shows that X CO depends on the excitation conditions. We find X CO ∼ T −1 kin n(H 2 ) 1/2 , as expected for virialized clouds.

A titanic interstellar medium ejection from a massive starburst galaxy at redshift 1.4

“…This is consistent with the molecular gas density properties of high-redshift disks 42 containing both low-and moderately highdensity gas and suggests that we are observing ISM-like material tidally stripped from the original merging galaxies. In the feedback-driven wind scenario we would instead expect highly excited, low density gas 110 .…”