The Kinematic Structure of Magnetically Aligned HI Filaments

TL;DR

This study characterizes the three-dimensional structure, magnetic alignment, and velocity gradients of HI filaments in a high Galactic latitude region, revealing that most do not show simple velocity gradients and are aligned with magnetic fields.

Contribution

It introduces a new 3D filament identification method and provides the first detailed analysis of their kinematic and magnetic properties in this region.

Findings

Most HI filaments are aligned with magnetic fields.

Only 15% of filaments show significant velocity gradients.

Typical velocity gradients are around 0.5 km/s/pc.

Abstract

We characterize the kinematic and magnetic properties of HI filaments located in a high Galactic latitude region ($165^\circ < \alpha < 195^\circ$ and $12^\circ < \delta < 24^\circ$). We extract three-dimensional filamentary structures using \texttt{fil3d} from the Galactic Arecibo L-Band Feed Array HI (GALFA-HI) survey 21-cm emission data. Our algorithm identifies coherent emission structures in neighboring velocity channels. Based on the mean velocity, we identify a population of local and intermediate velocity cloud (IVC) filaments. We find the orientations of the local (but not the IVC) HI filaments are aligned with the magnetic field orientations inferred from Planck 353 GHz polarized dust emission. We analyze position-velocity diagrams of the velocity-coherent filaments, and find that only 15 percent of filaments demonstrate significant major-axis velocity gradients with a median magnitude of 0.5 km s$^{-1}$ pc$^{-1}$, assuming a fiducial filament distance of 100 pc. We conclude that the typical diffuse HI filament does not exhibit a simple velocity gradient. The reported filament properties constrain future theoretical models of filament formation.