We present the first detection of molecular gas in the Cartwheel , the prototype of a collisional ring galaxy formed in the head-on encounter of two galaxies .
Until now , only very little atomic gas and no CO had been detected in the centre , where gas is theoretically expected to pile up .
Using the Swedish ESO Submm Telescope , we detected both ^ { 12 } CO ( 1–0 ) and ( 2–1 ) line emission towards the central position .
The line ratio and the line widths suggest that the CO ( 2–1 ) emission is sub-thermal and that the CO ( 1–0 ) emission arises within the central 22 ^ { \prime \prime } ( 13 kpc ) ; it is probably associated with the inner ring and nucleus .
We infer a mass of molecular gas ( H _ { 2 } ) of 1.5 to 6 10 ^ { 9 } M _ { \odot } , which is significantly higher than the \sim 10 ^ { 8 } M _ { \odot } of atomic gas within that region .
The low excitation of the gas , whether it is due to a low temperature or a low density , is consistent with the weak star-forming activity observed in the centre of the Cartwheel .