The large-scale atomic and molecular gas in the Circinus galaxy

We have used the Australia Telescope Compact Array (ATCA) and the Swedish-ESO Sub-millimetre Telescope (SEST) to map the large-scale atomic and molecular gas in the nearby (4 Mpc) Circinus galaxy. The ATCA H I mosaic of Circinus exhibits the warps in position angle and inclination revealed in the single-pointing image of Jones et al., both of which appear to settle beyond the inner 30 kpc which was previously imaged. The molecular gas has been mapped in both the CO J = 1 -> 0 and J = 2 -> 1 transitions down to a colunin density of N-H2 greater than or similar to 10(21) cm(-2) (3 sigma), where we derive a total molecular gas mass of MH2 approximate to 2 x 10(9) M-circle dot. Within a radius of 3 kpc, i.e. where CO was clearly detected, the molecular fractioll climbs steeply from approximate to 0.7 to unity (where N-H2 = 4 x 10(22) cm(-2), cf. N-H I = 10(21) cm(-2)) with proximity to the nucleus. Our H I mosaic gives an atornic gas mass of M-H I approximate to 6 x 10(9) M-circle dot, which is 70 per cent of the fully mapped single dish value. Combining the atomic and molecular gas masses gives a total gas mass of M-gas equivalent to M-H1 + M-H2 approximate to 1 x 10(10) M-circle dot, cf. the total dynamical mass of approximate to 3 x 10(11) M-circle dot within the inner 50 kpc ofour mosaiced image. The total neutral gas mass to dynamical mass ratio is therefore 3 per cent, consistent with the SAS3 (Third Small Astronomy Satellite) classification of Circinus. The high (molecular) gas mass fraction of M-H2/M-dyn approximate to 50 per cent found by Curran et al., only occurs close to the central approximate to 0.5 kpc and falls to less than or similar to 10 per cent within and outwith this region, allaying previous concerns regarding the validity of applying the Galactic N-H2/I-CO conversion ratio to Circinus. The rotation curve, as traced by both the H I and CO, exhibits a steep dip at approximate to 1 kpc, the edge of the atomic/molecular ring, within which the starburst is occurring. We find the atomic and molecular gases to trace different kinematical features and believe that the fastest part (greater than or similar to 130 km s(-1)) of the subkpc ring consists overwhelmingly of molecular gas. Beyond the inner kpc, the velocity climbs to settle into a solid body rotation approximate to 150 km s(-1) at 10kpc. Most of the starlight emanates frorn within this radius and so much of the dynamical mass, which remains climbing to the limit of our data (greater than or similar to 50 kpc), must be due to the dark matter halo.