Molecular Gas and Star Formation in the Cartwheel

Atacama Large Millimeter/submillimeter Array (ALMA) 12CO(J=1-0) observations are used to study the cold molecular ISM of the Cartwheel ring galaxy and its relation to HI and massive star formation (SF). CO moment maps find \((2.69\pm0.05)\times10^{9}\) M\(_{\odot}\) of H\(_2\) associated with the inner ring (72%) and nucleus (28%) for a Galactic I(CO)-to-N(H2) conversion factor (\(\alpha_{\rm CO}\)). The spokes and disk are not detected. Analysis of the inner ring's CO kinematics show it to be expanding (\(V_{\rm exp}=68.9\pm4.9\) km s\(^{-1}\)) implying an \(\approx70\) Myr age. Stack averaging reveals CO emission in the starburst outer ring for the first time, but only where HI surface density (\(\Sigma_{\rm HI}\)) is high, representing \(M_{\rm H_2}=(7.5\pm0.8)\times10^{8}\) M\(_{\odot}\) for a metallicity appropriate \(\alpha_{\rm CO}\), giving small \(\Sigma_{\rm H_2}\) (\(3.7\) M\(_{\odot}\) pc\(^{-2}\)), molecular fraction (\(f_{\rm mol}=0.10\)), and H\(_2\) depletion timescales (\(\tau_{\rm mol} \approx50-600\) Myr). Elsewhere in the outer ring \(\Sigma_{\rm H_2}\lesssim 2\) M\(_{\odot}\) pc\(^{-2}\), \(f_{\rm mol}\lesssim 0.1\) and \(\tau_{\rm mol}\lesssim 140-540\) Myr (all \(3\sigma\)). The inner ring and nucleus are H\(_2\)-dominated and are consistent with local spiral SF laws. \(\Sigma_{\rm SFR}\) in the outer ring appears independent of \(\Sigma_{\rm H_2}\), \(\Sigma_{\rm HI}\) or \(\Sigma_{\rm HI+H_2}\). The ISM's long confinement in the robustly star forming rings of the Cartwheel and AM0644-741 may result in either a large diffuse H\(_2\) component or an abundance of CO-faint low column density molecular clouds. The H\(_2\) content of evolved starburst rings may therefore be substantially larger. Due to its lower \(\Sigma_{\rm SFR}\) and age the Cartwheel's inner ring has yet to reach this state. Alternately, the outer ring may trigger efficient SF in an HI-dominated ISM.