A DISTURBED MOLECULAR DISC IN THE CIRCINUS GALAXY

We have used the Swedish ESO Submillimeter Telescope to observe the mo lecular gas in the Circinus galaxy using the CO(1-->0) transition as a tracer. The central region and major axis have been mapped and severa l other points were also observed. The gas in the galaxy is concentrat ed towards the nucleus, the peak being coincident with the radio/optic al core. The inclination of the molecular galactic disc is more compar able to that of the radio continuum than to that of the large-scale HI emission. Evidence for an anomalous spur structure pointing radially away from the galactic centre is presented, and may indicate a causal link between it and similar features seen in optical lines and radio c ontinuum. Our data suggest the presence of a central molecular ring or disc with radius 300+/-50 pc and a rotation velocity of about 200 km s(-1) (assuming i = 73 degrees). The dynamical mass of the nucleus is estimated to be no greater than 3.9 x 10(9) M.. Assuming that the dist ribution of gas varies smoothly in the outer regions, we calculate the mass of molecular gas in the galaxy to be at least M-mol = 1.1 x 10(9 ) M., and the star-forming efficiency to be 11 +/- 2 L.M.(-1). These r esults imply that Circinus is undergoing a massive central starburst w hich may be, at least partially, responsible for its extended minor ax is emission seen in several wavebands.