2025MNRAS.536.3507H

We present findings of 3D filamentary structures in the Smith Cloud, a high-velocity cloud (HVC) located at $l=38^{\circ }$, $b=-13^{\circ }$. These data represent the first detection of velocity-resolved 3D H I filaments within an HVC. We use data from the Galactic Arecibo L-Band Feed Array H I (GALFA-H I) along with our new filament detection algorithm, fil3d, to characterize these structures. In this paper, we also discuss how different input parameters affect the output of fil3d. We study filaments in the local interstellar medium (ISM) and compare them to those found in the Smith Cloud. Based on thermal line width estimations we find supporting evidence that the Smith Cloud filaments are part of its warm neutral medium. We also find a relationship between thermal line width and the $v_{\mathrm{LSR}}$ of the filaments. We study the plane-of-sky magnetic field as traced by Planck 353 GHz polarized dust emission along the line of sight and find none of our filament populations are aligned with this tracer of the magnetic field. This is likely related to their location close to dynamic processes in the Galactic Plane and/or the low column density of the filaments relative to emission in the Plane. The results show that 3D H I filaments are found in a wide range of Galactic environments and form through multiple processes.