2015ApJ...814L...1H

Atacama Large Millimeter/submillimeter Array ¹²CO(J = 1-0) observations are used to study the cold molecular ISM of the Cartwheel ring galaxy and its relation to H i and massive star formation (SF). CO moment maps find (2.69±0.05)x10⁹ M_☉ of H₂associated with the inner ring (72%) and nucleus (28%) for a Galactic I_CO-to- conversion factor (α_CO). The spokes and disk are not detected. Analysis of the inner ring's CO kinematics shows it to be expanding (V_exp= 68.9±4.9 km/s), implying an ≈70 Myr age. Stack averaging reveals CO emission in the starburst outer ring for the first time, but only where H i surface density (Σ_H i_) is high, representing M_☉for a metallicity-appropriate α_CO, giving small (3.7 M_☉/pc2), molecular fraction (f_mol= 0.10), and H₂depletion timescales (τ_mol≈ 50-600 Myr). Elsewhere in the outer ring M_☉/pc2, f_mol≲ 0.1 and τ_mol≲ 140-540 Myr (all 3σ). The inner ring and nucleus are H₂ dominated and are consistent with local spiral SF laws. Σ_SFRin the outer ring appears independent of Σ_H i_, or The ISM's long confinement in the robustly star-forming rings of the Cartwheel and AM0644-741 may result in either a large diffuse H₂component or an abundance of CO-faint low column density molecular clouds. The H₂content of evolved starburst rings may therefore be substantially larger. Due to its lower Σ_SFRand age, the Cartwheel's inner ring has yet to reach this state. Alternately, the outer ring may trigger efficient SF in a H i-dominated ISM.