Molecular Gas Filaments and Fallback in the Ram Pressure Stripped Coma Spiral NGC 4921

We investigate the effects of ram pressure on the molecular interstellar medium (ISM) in the disk of the Coma cluster galaxy NGC 4921 via high-resolution CO observations. We present 6″ resolution CARMA CO(1−0) observations of the full disk, and 0.″4 resolution Atacama Large Millimeter/submillimeter Array CO(2−1) observations of the leading quadrant, where ram pressure is strongest. We find evidence for compression of the dense ISM on the leading side, spatially correlated with intense star formation activity in this zone. We also detect molecular gas along kiloparsec-scale filaments of dust extending into the otherwise gas stripped zone of the galaxy, seen in Hubble Space Telescope images. We find the filaments are connected kinematically as well as spatially to the main gas ridge located downstream, consistent with cloud decoupling inhibited by magnetic binding, and inconsistent with a simulated filament formed via simple ablation. Furthermore, we find several clouds of molecular gas ∼1–3 kpc beyond the main ring of CO that have velocities that are blueshifted by up to 50 km s−1 with respect to the rotation curve of the galaxy. These are some of the only clouds we detect that do not have any visible dust extinction associated with them, suggesting that they are located behind the galaxy disk midplane and are falling back toward the galaxy. Simulations have long predicted that some gas removed from the galaxy disk will fall back during ram pressure stripping. This may be the first clear observational evidence of gas re-accretion in a ram pressure stripped galaxy.