Molecular gas in the Cartwheel Galaxy

We present the first detection of molecular gas in the Cartwheel, the proto type of a collisional ring galaxy formed in the head-on encounter of two ga laxies. 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 Sw edish ESO Submm Telescope, we detected both (CO)-C-12(1-0) and (2-1) line e mission towards the central position. The line ratio and the line widths su ggest that the CO(2-1) emission is sub-thermal and that the CO(1-0) emissio n arises within the central 22 " (13 kpc); it is probably associated with t he inner ring and nucleus. We infer a mass of molecular gas (H-2) of 1.5 to 6 10(9) M., which is significantly higher than the similar to 10(8) M. 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 sta r-forming activity observed in the centre of the Cartwheel.