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 .
( 30 ) , 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 \rightarrow 0 and J = 2 \rightarrow 1 transitions down to a column density of N _ { { H } _ { 2 } } \ga 10 ^ { 21 } cm ^ { -2 } ( 3 \sigma ) , where we derive a total molecular gas mass of M _ { H _ { 2 } } \approx 2 \times 10 ^ { 9 } M _ { \odot } .
Within a radius of 3 kpc , i.e .
where CO was clearly detected , the molecular fraction climbs steeply from \approx 0.7 to unity ( where N _ { { H } _ { 2 } } = 4 \times 10 ^ { 22 } cm ^ { -2 } , cf . N _ { HI } = 10 ^ { 21 } cm ^ { -2 } ) with proximity to the nucleus .
Our H i mosaic gives an atomic gas mass of M _ { HI } \approx 6 \times 10 ^ { 9 } M _ { \odot } , which is 70 % of the fully mapped single dish value .
Combining the atomic and molecular gas masses gives a total gas mass of M _ { gas } \equiv M _ { HI } + M _ { H _ { 2 } } \approx 1 \times 10 ^ { 10 } M _ { \odot } , cf .
the total dynamical mass of \approx 3 \times 10 ^ { 11 } M _ { \odot } within the inner 50 kpc of our mosaiced image .
The total neutral gas mass to dynamical mass ratio is therefore 3 % , consistent with the SAS3 classification of Circinus .
The high ( molecular ) gas mass fraction of M _ { H _ { 2 } } / M _ { dyn } \approx 50 % found previously ( 11 ) , only occurs close to the central \approx 0.5 kpc and falls to \stackrel { < } { { } _ { \sim } } 10 % within and outwith this region , allaying previous concerns regarding the validity of applying the Galactic N _ { H _ { 2 } } / I _ { CO } conversion ratio to Circinus .
The rotation curve , as traced by both the H i and CO , exhibits a steep dip at \approx 1 kpc , the edge of the atomic/molecular ring , within which the star-burst is occurring .
We find the atomic and molecular gases to trace different kinematical features and believe that the fastest part ( \stackrel { > } { { } _ { \sim } } 130 km s ^ { -1 } ) of the sub-kpc ring consists overwhelmingly of molecular gas .
Beyond the inner kpc , the velocity climbs to settle into a solid body rotation of \approx 150 km s ^ { -1 } at \stackrel { > } { { } _ { \sim } } 10 kpc .
Most of the starlight emanates from within this radius and so much of the dynamical mass , which remains climbing to the limit of our data ( \ga 50 kpc ) , must be due to the dark matter halo .