CO (3 → 2) mapping and gas excitation in the core of M82

We have observed the CO (3 → 2) emission in the central region of M82 with a 14″ FWHM beam using the Berkeley SIS receiver system at the 15 m James Clerk Maxwell Telescope on Mauna Kea. The map clearly shows the double-peaked structure seen at other wavelengths, that has been interpreted as a circumnuclear ring. We report also on the first ¹³CO (3 → 2) detections in M82. At the positions of the nucleus and the ring-peaks, the I_co/I_13CO (3 → 2) ratio is ∼ 10. Using a large velocity gradient (LVG) model, the six observed lines (the three lowest transitions of ¹²CO and ¹³CO) allow a rather direct determination of the properties of the molecular gas in M82. The ratios of these lines are rather unusual and cannot be reproduced with a standard one-component model and an abundance ratio [¹²CO]/[ ¹³CO] of 70 (solar), nor by simple mixtures of optically thin and thick CO components. On the other hand, using a one-component model a significantly better fit to all of the CO lines can be obtained if the abundance ratio, [¹²CO]/[¹³CO], is lowered to values of 25 or below. Such values are similar to the abundance ratio observed in the center of the Milky Way. For an assumed abundance ratio of 20, our analysis suggests that the molecular gas in the central region of M82 is both hotter and denser on average than in the Milky Way.