CO at the centre of M82

N. Neininger , M. Guélin , U. Klein , S. Garcıa-Burillo , and R. Wielebinski
Radioastronomisches Institut der Universität Bonn, Auf dem Hügel 71, D-53121 Bonn, Germany
MPIfR, Auf dem Hügel 69, D-53121 Bonn, Germany
IRAM, 300 rue de la Piscine, F-38406 St. Martin d'Héres, France
OAN, Apartado 1143, E-28800 Alcalá de Henares, Spain
Abstract: Using the IRAM interferometer, we have observed the nearby starburst galaxy M82 in the 1 0 line of CO and in the continuum at resolution ( pc). The spatial distribution of the CO line (Fig. 5) shows the same gross features as the CO(1 0) map of Shen and Lo (1995) - though with different relative intensities - namely two `lobes' and a weaker `central' source. In comparison, the `lobes' are more conspicuous and the central source is fainter in CO than in CO. These lobes probably mark the end of a nuclear bar: the velocity field observed around the nucleus shows a very steep gradient (140 kms over 75pc), the usual signpost of a bar. The dynamical center coincides with the strong IR peak and is shifted 6 north-east of the `central' CO peak.

The distribution and kinematics of the molecular gas is better described by gas flowing radially inward, rather than by a rotating molecular ring. The double-lobed morphology of the CO emission is explained in terms of dissociation of CO molecules by the intense and energetic radiation field in the central 250 pc region of M82. Denser clouds as traced in the HCN line are better shielded against this radiation and have survived, thus making the contrast between the lobes and the central region much smaller in this line. This interpretation is in line with the bar scenario that ensued from previous observations in the near infrared.

A giant arc has been identified which happens to coincide with the most luminous compact radio source in M82. It lies in a region between the central and the western lobe, which is void of CO gas. The whole region coincident with it is featured by warmer gas, strong free-free and CI emission as well as by an enhanced cosmic ray production rate. This 130pc region must be a location of strong ionization and dissociation, except for gas locked in dense clouds.

 
Figure 5: The figure shows the integrated CO(1-0) emission of M82. The stars mark the positions of the point sources detected at 6cm, with their size scaled to the logarithm of the flux. Open stars are most probably SNR's; the oblique cross marks the position of the NIR center which is close to the dynamical center of M82. In the lower left the derived rotation curve is shown together with the derived pattern speed and the locations of the Lindblad resonances. The kinematical signature indicates the presence of a bar. The upper right shows a channel map at the intrinsic rotation speed of -65 kms . The star marks the strongest SNR wich lies in the middle of a giant bubble with a diameter of about 100pc. This bubble is also prominent in other tracers including the low-frequency radio continuum.

Astronomy & Astrophysics in press. Preprint requests: N. Neininger, nneini@astro.uni-bonn.de