FAST Observations of Filamentary and Compact H i Structure in the Magellanic Stream IV Field

Abstract The Magellanic Stream (MS) is believed to have formed from gas removed from the Large and Small Magellanic Clouds through tidal forces and hydrodynamic interactions with the Milky Way’s gaseous halo. It provides an important laboratory for studying how stripped gas fragments, mixes, and evolves in a circumgalactic environment. In this work, we present H i observations of a 3.8° × 2.2° field in the MS IV region, using the data from the Commensal Radio Astronomy FasT Survey (CRAFTS). The total H i mass in the analyzed field is ≃5.3 × 10 6 ( d /120 kpc) 2 M ⊙ , where d is the distance to the MS IV gas. The data resolve the emission into three coherent filamentary H i structures with related but distinct velocity trends. To characterize the H i structures and study potential multiphase gas, we adopt a Gaussian decomposition procedure to identify and reconstruct sources. Our results indicate that the field is dominated by one major filamentary H i complex, together with several smaller kinematic clump-like components. Among these identified sources, only one source likely shows multiple velocity components. Because several sources appear spatially overlapped in projection, we further examine their apparent overlap regions using position–velocity ( P - V ) diagrams. P - V diagrams across the apparent overlap region show no clear intermediate-velocity bridge or V-shaped structure, favoring line-of-sight projection over direct cloud–cloud collision. These results demonstrate the value of deep, high-angular-resolution H i observations for resolving faint emission, compact morphologies, and kinematic structure in the MS.