Molecular gas and star formation in the cartwheel

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) × 10⁹ M_⊙ of H₂ associated with the inner ring (72%) and nucleus (28%) for a Galactic I_CO-to-$N_{H_2}$ 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⁻¹), 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 $M_{H_2}=(7.5\pm 0.8)\times {10}^8$ M_⊙ for a metallicity-appropriate α_CO, giving small ${\Sigma }_{H_2}$ (3.7 M_⊙ pc⁻²), molecular fraction (f_mol = 0.10), and H₂ depletion timescales (τ_mol ≈ 50–600 Myr). Elsewhere in the outer ring ${\Sigma }_{H_2}\lesssim 2$ M_⊙ pc⁻², 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_SFR in the outer ring appears independent of ${\Sigma }_{H_2},$ Σ_{H i}, or ${\Sigma }_{HI+H_2}.$ The ISM's long confinement in the robustly star-forming rings of the Cartwheel and AM0644-741 may result in either a large diffuse H₂ component or an abundance of CO-faint low column density molecular clouds. The H₂ content of evolved starburst rings may therefore be substantially larger. Due to its lower Σ_SFR and 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.