The cold neutral medium in filaments at high Galactic latitudes

TL;DR

This study reveals that the cold neutral medium at high Galactic latitudes predominantly resides in filamentary structures, with absorption features closely associated with FIR and HI filaments, and characterizes their physical and turbulent properties.

Contribution

It demonstrates that all HI absorption lines at high Galactic latitudes are linked to filamentary structures, providing detailed physical parameters and turbulence analysis of the CNM within these filaments.

Findings

57% of absorption positions coincide with FIR/HI caustics

CNM filament velocity deviations are due to local turbulence

Average turbulent velocity dispersion of CNM is 2.48-3.9 km/s

Abstract

Context. The HI distribution at high Galactic latitudes is found to be filamentary and closely related to the far infrared (FIR) in caustics with coherent velocity structures. These structures trace the orientation of the magnetic field lines. Aims. Recent absorption observations with the Australian SKA Pathfinder Telescope have led to major improvements in our understanding of the physical properties of the cold neutral medium (CNM) at high Galactic latitudes. We use these results to explore how far the physical state of the CNM may be related with caustics in HI and FIR. Methods. We trace filamentary FIR and HI structures and probe the absorption data for coincidences in position and velocity. Results. 57 percent of the absorption positions are associated with known FIR/HI caustics, filamentary dusty structures with a coherent velocity field. The remaining part of the absorption sample is coincident in position and velocity with genuine HI filaments that are closely related to the FIR counterparts. Thus, within the current sensitivity limitations, all of the positions with HI absorption lines are associated with filamentary structures in FIR and/or HI. We summarize physical parameters for then CNM along filaments in the framework of filament velocities that have been determined from a Hessian analysis of FIR and HI emission data. Velocity deviations between absorption components and filament velocities are due to local turbulence and we determine for the observed CNM an average turbulent velocity dispersion of 2.48 < delta_vturb < 3.9 km/s. The CNM has a mean turbulent Mach number of Mt = 3.4 +/- 1.6 km/s. Conclusions. Most, if not all, of the CNM in the diffuse ISM at high Galactic latitudes is located in filaments, identified as caustics with the Hessian operator.