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

TL;DR

This study uses FAST’s high-resolution HI data to resolve HVC AC-III into parabolic, ram-pressure–confined droplets (D1–D6) with nearly constant line widths around $ $20~km s^{-1}$, whose global velocity spread arises from clump-to-clump motions. A steady-state, ram-pressure–driven density model reproduces the observed parabolic boundaries and exponential density profiles, with a tip density near $ _{ m tip}\sim 2~\mathrm{cm^{-3}}$ implying an ambient WIM density of $n_{ m ext}\sim 10^{-3}~\mathrm{cm^{-3}}$ at the cloud’s location, consistent with a few kpc-scale height above the plane. Deviations from symmetry and the presence of strips, rings, and holes indicate rich internal dynamics, including inter-droplet interactions and persistent internal loops that can drive secondary turbulence. An intermediate-velocity component exhibits shell-like morphology aligned with the head, suggesting pressure interactions mediated by the WIM. Overall, AC-III is interpreted as a cloud entering the Galactic WIM, sculpted by ram pressure into droplets, providing a valuable case to study structure formation, turbulence, and ambient-medium physics in HVCs.

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.

Figures (18)

• Figure 1: HI4PI 2016AA...594A.116H moment-0 map of AC-III, integrated over $-250$ to $-150$ km s$^{-1}$. White squares indicate the four OTF fields observed with FAST (see Sect. \ref{['sec_fastobs']}). The dotted white line outlines the full FAST coverage, including the noisy margins (see the upper-left panel of Fig. \ref{['noisetime']}). The red line traces the global trend of the ram-pressure--induced parabolic front, identified from a joint analysis of the HI4PI and FAST images. The field center of the white dashed box (FAST sky coverage) has Galactic coordinates $(\ell,b)=(189.445\degr,-29.636\degr)$. The orange arrow denotes the direction of increasing Galactic latitude, $\Delta b$, i.e., toward the Galactic plane, since the target lies in the southern Galactic hemisphere. The black solid and dashed lines indicate two position–velocity cuts, labeled PV1 and PV2, respectively, used for the analysis presented in Sect. \ref{['sec_IVC']}.
• Figure 2: Moment maps of the FAST HI emission of AC-III, integrated over $-250$ to $-160~\mathrm{km~s^{-1}}$. The moment 0 map ($M_0$ or $I$, left) is shown without thresholding, while the velocity ($M_1$ or $V$, middle) and line-width ($\Delta V$, Eq. \ref{['eqM2DV']}, right) maps use only channels above $3.5\sigma$. The cyan outline in the middle and right panels marks the FAST coverage (Figure \ref{['fig:HI4PImom0']}). Spectra at the four black crosses in the right panel, labeled p1--p4 from left to right, are shown in Figure \ref{['fig_speexample']}.
• Figure 3: Left: Mean spectrum (blue) of the FAST-covered region of AC-III, together with a three-component Gaussian fit (orange). The Gaussian fitting results are shown in the upper right corner, including the peak brightness temperature ($T_{\rm peak}$), the central velocity ($V$), and the FWHM line width ($\Delta$) of each component. Right: Same as the left panel but for the spectra at positions p1--p4 (see right panel of Figure \ref{['fig_mommaps']}). The fitted FWHM line widths are labeled. For clarity, the spectrum at p1 has been divided by a factor of five.
• Figure 4: Moment 0 and 1 maps, similar to those in Figure \ref{['fig_mommaps']}, with curves highlighting droplet-like features and ellipses indicating ring-like (or hole-like) structures. The colorbars and coordinate ticks are the same as in the corresponding panels of Figure \ref{['fig_mommaps']} and are therefore not labeled. The blue curve (D1) in the upper panel corresponds to the red curve in Figure \ref{['fig:HI4PImom0']}.
• Figure 5: Channel maps of the HI emission of AC III observed with FAST. Each panel is integrated over 5 km s$^{-1}$, with consecutive panels offset by the same velocity step. The central velocity of each interval (in km s$^{-1}$) is indicated by blue text in the upper-left corner of each panel. All panels share the same colorbar. The coordinate ticks are identical to those in Figure \ref{['fig_mommaps']}, and the cyan line outlines the region covered by the FAST observations.