1998A&A...339..737N

Using the IRAM interferometer, we have observed the nearby starburst galaxy M82 with a 4.2" resolution (≃70pc) in the 1->0 line of ¹³CO and in the λ 2.6-mm continuum. The spatial distribution of the ¹³CO line shows the same gross features as the ¹²CO(1->0) map of Shen & Lo (1995ApJ...445L..99S) namely two lobes and a compact central source, though with different relative intensities. The lobes are more conspicuous and the central source is fainter in ¹³CO than in ¹²CO. The velocity field observed around the nucleus shows a very steep gradient (140km/s over 75pc), which is very probably caused by the stellar bar visible in the near infrared. The dynamical centre coincides with the IR peak and is shifted 6'' north-east of the compact ¹³CO source. The two CO lobes appear to be associated with the ends of the bar and not with a molecular ring, as usually assumed. They are probably shaped by the strong UV radiation from the central region. ¹³CO must be more photodissociated than the self-shielded ¹²CO molecules in the central ∼250pc region, which may explain the relative weakness of the ¹³CO central source. A 130pc-wide bubble of molecular gas has been identified, which happens to host the most luminous compact radio source in M82. It lies 120pc west of the IR peak between the central source and the western lobe and seems characterized by warmer gas, strong UV radio free-free radiation, and an enhanced cosmic ray production rate.