Modeling the Accretion of High-velocity Clouds from a Rotating Halo

Abstract High-velocity clouds (HVCs) are a major fuel reservoir for star formation in the Galactic disk. Determining their origin and kinematics is thus crucial for understanding Galactic evolution. In this paper, we employ simple test-particle simulations to model HVC kinematics, generating line-of-sight velocity maps and probability density functions for comparison with observational results. We find that models assuming low angular momentum and an initial scale of tens of kiloparsecs successfully reproduce the observed kinematic trends for both blueshifted and redshifted components. This consistency may support the dominance of intermediate-halo dynamics (tens-of-kiloparsec scale) in regulating Galactic evolution, consistent with HVC formation via thermal instability in metal-polluted gas in the halo. Furthermore, by considering the entire bulk mass involved in the continuous accretion process—including diffuse or ionized components that often escape direct observation—our theoretical estimates yield a total mass accretion rate of several M ⊙ yr −1 . This indicates that HVC accretion has the potential to supply a sufficient amount of gas to the Galactic disk to sustain ongoing star formation over several Gyr. Our findings suggest that the Galactic baryon cycle and disk evolution are governed by dynamics within the intermediate halo, providing key kinematic constraints for future magnetohydrodynamical simulations that resolve spatial structures of high-velocity clouds.