2025ApJ...978..111E

We present the large-scale distribution and kinematics of cold molecular gas across the compact galaxy group Stephan's Quintet, based on CO(2-1) observations performed with the Atacama Compact Array (ACA) and CO(1-0) data from the Combined Array for Research in Millimeter-wave Astronomy (CARMA). We find coherent structures of molecular gas associated with the galaxies and intragroup medium, which follow the distribution of warm H₂ previously seen with the James Webb Space Telescope (JWST). CO is associated with a ridge of shocked gas that crosses the galaxy group, and with a spiral arm of the intruding galaxy NGC 7318b, which interacts with the intragroup medium along the ridge. Although the ridge contains widespread shocks, turbulent gas, and warm H₂, the CO lines are narrower than elsewhere in Stephan's Quintet (FWHM ∼ 25-65 km s^–1), indicative of settled cold gas. At a distinctly different velocity, CO is found in the active galaxy NGC 7319 and northern star-forming region SQ-A. A bridge of turbulent molecular gas connects NGC 7319 with the ridge, covering a gap of ∼700 km s^–1 between these structures. The gas excitation ranges from ${L}_{\left.{\rm{CO}}(2-1\right)}^{{\prime} }$/${L}_{\left.{\rm{CO}}(1-0\right)}^{{\prime} }$ ∼ 0.3 in the bridge and SQ-A, to ∼0.5 along the ridge, to near unity in the center of NGC 7319. We also detect either a molecular outflow or turbulent molecular gas associated with the radio source in NGC 7319. These ACA data are part of a program with the Atacama Large Millimeter/submillimeter Array and JWST to study the physics of molecular gas from the largest to the smallest scales across the intragroup medium of Stephan's Quintet.