ASTRONOMY AND ASTROPHYSICS 13 CO at the centre of M 82

Using the IRAM interferometer, we have observed the nearby starburst galaxy M 82 with a 4. ′′2 resolution (' 70 pc) 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 (1995), 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 (140 km s−1 over 75 pc), 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 ∼250 pc region, which may explain the relative weakness of the 13CO central source. A 130 pc-wide bubble of molecular gas has been identified, which happens to host the most luminous compact radio source in M 82. It lies 120 pc 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.