Molecular hydrogen 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: 12 March 2025
Accepted: 21 November 2025

Abstract

Context. Neutral atomic hydrogen (H I) absorption lines can be used to probe the cold neutral medium (CNM) at high Galactic latitudes. Cold H I with a significant optical depth from the GASKAP-H I survey is found to be located predominantly if not exclusively within filamentary structures that can be identified as caustics with the Hessian operator. Most of these H I filaments (57%) are also observable in the far-infrared (FIR) and trace the orientation of magnetic field lines.

Aims. We considered whether molecular hydrogen (H₂) might also be preferentially associated with CNM filaments.

Methods. We analyzed 241 H₂ absorption lines against stars and determined whether the lines of sight intersected H I or FIR filaments. Using Far Ultraviolet Spectroscopic Explorer (FUSE) H₂ data in the velocity range −50 < v_LSR < 50 km s⁻¹, we traced 65 additional H₂ lines for filamentary H I and FIR structures in velocity and probed the H₂ absorption for coincidences in position and velocity.

Results. For 305 out of 306 positions, the lines of sight with H₂ absorption intersect H I filaments. In 120 cases, there is also evidence for a correlation with dusty FIR filaments. All of the 65 available sight lines with known velocities intersect H I filaments. In 64 cases, the H₂ velocities are consistent with H I filament velocities. For FIR filaments, an agreement is found for only 13 out of 14 H₂ absorption lines.

Conclusions. For the majority of H₂ absorption lines, there is evidence that H₂ is associated with cold H I filaments. Evidence of an association with FIR filaments is less compelling. Confusion along the line of sight limits the detectability of FIR filaments. For a comparable degree of UV excitation in the disk and lower Galactic halo, the formation rate of H₂ appears to be enhanced in H I filaments with increased CNM densities.