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

Abstract

FAST HI observations reveal unprecedented internal structures within the high-velocity cloud (HVC) AC-III, which consists of several coherent subclumps (D1--D6). These subclumps exhibit nearly constant line widths of $\sim 20$ km/s across a velocity ($v_{\rm LSR}$) range of $-220$ to $-180$ km/s. They display a parabolic morphology consistent with ram-pressure-confined droplets whose heads are oriented toward the Galactic plane. A steady-state analytic model can reproduce both the morphologies and the exponential density profiles of these droplets. The density at the droplet heads reaches $\sim 2$ cm$^{-3}$, implying an ambient gas density of $\sim 10^{-3}$ cm$^{-3}$, which matches the conditions of the Galactic warm ionized medium (WIM). Deviations from axial symmetry, alongside rich dynamic patterns (ridges, rings, and cavities), indicate complex internal dynamics within AC-III. These patterns may be caused by fluid loops induced by interactions between neighboring droplets. Additionally, we find an intermediate-velocity component ($-150$ to $-100$ km s$^{-1}$) showing a distinct shell-like morphology that envelops the head of AC-III, suggesting a non-contact interaction possibly mediated by the WIM. Overall, we suggest that AC-III is actively entering the Galactic WIM layer and being sculpted by external drag into a droplet-like morphology, providing an ideal laboratory for investigating the evolution of halo gas infalling toward the Galactic plane.