Letter to the Editor

The cold neutral medium in filaments at high Galactic latitudes

Argelander-Institut für Astronomie, University of Bonn, Auf dem Hügel 71, 53121 Bonn, Germany

^⋆ Corresponding author; pkalberla@astro.uni-bonn.de

Received: 27 October 2024
Accepted: 29 January 2025

Abstract

Context. The H I distribution at high Galactic latitudes has been found to be filamentary and closely related to the far infrared (FIR) in caustics with coherent velocity structures. These structures trace the orientation of magnetic field lines.

Aims. Recent absorption observations with the Australian SKA Pathfinder Telescope have led to major improvements in the 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 H I and FIR.

Methods. We traced filamentary FIR and H I structures and probed the absorption data for coincidences in position and velocity.

Results. Of the absorption positions, 57% are associated with known FIR/H I caustics, filamentary dusty structures with a coherent velocity field. The remaining part of the absorption sample is coincident in position and velocity with genuine H I filaments that are closely related to the FIR counterparts. Thus, within the current sensitivity limitations, all the positions with H I absorption lines are associated with filamentary structures in FIR and/or H I. We summarize the physical parameters for the CNM along filaments in the framework of filament velocities v_fil that have been determined from a Hessian analysis of FIR and H I 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 < δv_turb < 3.9 km s⁻¹. The CNM has a mean turbulent Mach number of M_t = 3.4 ± 1.6 km s⁻¹.

Conclusions. Most, if not all, of the CNM in the diffuse interstellar medium at high Galactic latitudes is located in filaments, identified as caustics with the Hessian operator.