The Large-Scale Atomic and Molecular Gas in the Circinus Galaxy

Abstract

We have used the ATCA and the SEST to map the large-scale atomic and molecular gas in the nearby Circinus galaxy. The HI mosaic of Circinus exhibits the warps in position angle and inclination revealed in the single-pointing image, 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 transitions, where we derive a total molecular gas mass of ~2e9 Mo. Within a radius of 3 kpc, i.e. where CO was clearly detected, the molecular fraction climbs steeply from ~0.7 to unity with proximity to the nucleus. Our HI mosaic gives an atomic gas mass of ~6e9 Mo which is 70% of the fully mapped single dish value. 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 found previously, only occurs close to the central ~0.5 kpc and falls to < 10% within and outwith this region, allaying previous concerns regarding the validity of applying the Galactic conversion ratio to Circinus. The rotation curve, as traced by both the HI and CO, exhibits a steep dip at ~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 of the sub-kpc ring consists overwhelmingly of molecular gas. Beyond the inner kpc, the velocity climbs to settle into a solid body rotation at >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 (>50 kpc), must be due to the dark matter halo.