Ram pressure shaping HVC droplets -- FAST HI observations of HVC AC-III and theoretical interpretation

Abstract

FAST HI observations reveal unprecedented internal structures of the high-velocity cloud AC-III, which is found to consist of several coherent subclumps (D1-D6) with nearly constant line widths of $\sim 20~\mathrm{km~s^{-1}}$, while the global velocity spread ranges from $-220$ to $-180~\mathrm{km~s^{-1}}$. These subclumps exhibit parabolic morphologies, consistent with ram-pressure-confined droplets, with their heads tending to point toward the Galactic plane. A steady-state model reproduces both the morphology and the observed exponential density profiles. The tip density reaches $\sim 2~\mathrm{cm^{-3}}$, implying an ambient medium density of $\sim 10^{-3}~\mathrm{cm^{-3}}$, in agreement with the Galactic warm ionized medium at a distance of $\sim 5$~kpc. Deviations from symmetric droplet shapes, along with internal patterns such as strip-like ridges, rings, and holes, indicate rich internal dynamics. In particular, the observations are consistent with fluid loops forming inside the droplets in response to interactions between neighboring subclumps. These loops can generate ring-like dynamic patterns and drive secondary turbulence, sustaining long-lived internal motions. An intermediate-velocity component ($-150$ to $-100~\mathrm{km~s^{-1}}$) exhibits a shell-like morphology aligned with the head of AC-III, possibly shaped by pressure interactions mediated by the WIM. Overall, we suggest that HVC AC-III is entering the Galactic WIM layer and being sculpted by ram pressure into a droplet-like morphology, providing a valuable case for studying the structure formation, turbulence origin, and dynamic evolution of HVCs, as well as the physical properties of the ambient medium.