Star Formation in Circumgalactic High-Velocity Cloud

Abstract The accretion of metal-poor gas is critical to sustaining star formation and regulating the galaxy's baryon cycle. High-velocity clouds (HVCs) are a key source of such gas, yet their fundamental properties remain poorly understood due to the absence of stellar associations. In particular, the origin and evolution of their multiphase gas content has lacked direct observational constraints. Here we report the discovery of a 11 Myr-old binary open cluster with subsolar metallicity in HVC Complex H. From their locations and kinematics, we infer that the cloud is colliding with ambient gas in the outer Milky Way, producing pronounced Kelvin-Helmholtz instability features. Within this interacting system, the very high-velocity cold cores (local standard of rest velocity vlsr = -181km s⁻¹) remain shielded by a warmer envelope—a manifestation of the “peloton effect” that protects them from external ram pressure stripping. Intriguingly, these shielded cores subsequently collide with compact warm structures within the same HVC system, triggering massive star formation. These internal multiphase dynamics also produce the cometary morphology in the cold cores, whose axis aligns precisely with the clusters' motion. This binary cluster thus serves as both a descendant of its parent HVC and a potential progenitor of high-velocity stellar streams.