1998A&A...338..863C

The Circinus galaxy, a type 2 Seyfert, is suspected to have a molecular gas ring structure with a diameter of ∼500pc. We have mapped the lowest three rotational transitions of ¹²CO in the central region of the galaxy, and upon detailed examination, the data suggest that a fraction of the molecular gas is orbiting the galaxy's nucleus in the form of a ring. There also appears to exist an associated molecular outflow which is coincident with the previously observed ionisation cone. In order to determine the geometry and kinematics of the ring, we have modelled various configurations, obtaining an initial estimate from the MEM de-convolved data. The model takes defined ring parameters, i.e. velocities across the ring at various radii, the inclination angle and the intensity of emission, and convolves these with a synthesised telescope beam in order to regenerate the observed spectra. The results suggest that the ring, which has the same position angle as the main galaxy body, has an outer radius of >600pc and orbits with non-Keplerian rotation around a central dynamical mass of ≃3x10⁸M_☉. We also estimate the dynamical mass within the central 560 pc to be ≃3x10⁹M_☉ i.e. only double the total molecular mass calculated from the intensity of CO emission, indicating that use of the Galactic N_H2/I_CO conversion ratio may possibly cause us to overestimate the molecular mass in this nucleus.