Local H I filaments driven by a small-scale dynamo

Unraveling the velocities and tangling of dusty magnetized structures

¹ Argelander-Institut für Astronomie, Auf dem Hügel 71, 53121 Bonn, Germany
e-mail: pkalberla@astro.uni-bonn.de
² Tartu Observatory, University of Tartu, 61602 Tõravere, Tartumaa, Estonia

Received: 1 January 2021
Accepted: 7 August 2021

Abstract

Context. H I filaments are closely related to dusty magnetized structures that are observable in the far infrared (FIR). Recently it was proposed that the coherence of oriented H I structures in velocity traces the line of sight magnetic field tangling.

Aims. We study the velocity-dependent coherence between FIR emission at 857 GHz and H I on angular scales of 18′.

Methods. We use HI4PI H I data and Planck FIR data and apply the Hessian operator to extract filaments. For coherence, we require that local orientation angles θ in the FIR at 857 GHz along the filaments be correlated with the H I.

Results. We find some correlation for H I column densities at |v_LSR| < 50 km s⁻¹, but a tight agreement between FIR and H I orientation angles θ exists only in narrow velocity intervals of 1 km s⁻¹. Accordingly, we assign velocities to FIR filaments. Along the line of sight these H I structures show a high degree of the local alignment with θ, as well as in velocity space. Interpreting these aligned structures in analogy to the polarization of dust emission defines an H I polarization. We observe polarization fractions of up to 80%, with averages of 30%. Orientation angles θ along the filaments, projected perpendicular to the line of sight, are fluctuating systematically and allow a characteristic distribution of filament curvatures to be determined.

Conclusions. Local H I and FIR filaments identified by the Hessian analysis are coherent structures with well-defined radial velocities. H I structures are also organized along the line of sight with a high degree of coherence. The observed bending of these structures in the plane of the sky is consistent with models for magnetic field curvatures induced by a Galactic small-scale turbulent dynamo.