Star formation in the circumgalactic high-velocity cloud Complex H

The accretion of metal-poor gas sustains galactic star formation. In the Milky Way, this process is fuelled by high-velocity clouds (HVCs), but their fundamental properties have remained elusive in the absence of stellar tracers. Here we report a binary open cluster within HVC Complex H. With an age of 11.2 ± 0.6 Myr and a subsolar metallicity of $$0.0{5}_{-0.02}^{+0.05}$$ Z⊙, the clusters provide a direct stellar distance anchor to the cloud at 13.8 ± 0.6 kpc. Their proper motions indicate that Complex H is on a prograde, south-to-north orbit through the outer Galactic disk. The resulting interaction produces a ‘slow–fast–slow’ velocity gradient, with the cloud’s outer layers decelerating as they merge into the disk. Orbit integration suggests that the clusters formed from an internal cloud–cloud collision. This triggering mechanism implies that other HVCs could similarly produce high-velocity stars. The scarcity of previous stellar detections in HVCs is explained by the rapid escape of young stars (