[Paper Review] Multi-tracer analysis of straight depolarisation 
 canals in the surroundings of the 3C 196 field

Luka Turić, Vibor Jelić+7 more|arXiv (Cornell University)|Jan 1, 2021

Astrophysics and Star Formation StudiesPhysics and Astronomy88 references15 citations

TL;DR

This study extends multi-tracer Faraday tomography of LOFAR data to three fields around the 3C 196 radio source, using rolling Hough transforms to align depolarisation canals with HI filaments and dust-polarised magnetic fields. It presents the first direct distance estimate to depolarisation canals—found at ~200 pc—indicating they trace the edge of the Local Bubble where the magnetic field orientation changes abruptly.

ABSTRACT

Context. Faraday tomography of a field centred on the extragalactic point source 3C 196 with the LOw Frequency ARray (LOFAR) revealed an intertwined structure of diffuse polarised emission with straight depolarisation canals and tracers of the magnetised and multi-phase interstellar medium (ISM), such as dust and line emission from atomic hydrogen (HI). Aims: This study aims at extending the multi-tracer analysis of LOFAR data to three additional fields in the surroundings of the 3C 196 field. For the first time, we study the three-dimensional structure of the LOFAR emission by determining the distance to the depolarisation canals. Methods: We used the rolling Hough transform to compare the orientation of the depolarisation canals with that of the filamentary structure seen in HI, and based on starlight and dust polarisation data, with that of the plane-of-the-sky magnetic field. Stellar parallaxes from Gaia complemented the starlight polarisation with the corresponding distances. Results: Faraday tomography of the three fields shows a rich network of diffuse polarised emission at Faraday depths between − 10 and + 15 rad m−2. A complex system of straight depolarisation canals resembles that of the 3C 196 field. The depolarisation canals align both with the HI filaments and with the magnetic field probed by dust. The observed alignment suggests that an ordered magnetic field organises the multiphase ISM over a large area (~20°). In one field, two groups of stars at distances below and above 200 pc, respectively, show distinct magnetic field orientations. These are both comparable with the orientations of the depolarisation canals in the same field. We conclude that the depolarisation canals likely trace the same change in the magnetic field as probed by the stars, which corresponds to the edge of the Local Bubble.

Motivation & Objective

• To extend multi-tracer analysis of LOFAR Faraday tomography beyond the 3C 196 field to three adjacent fields.
• To determine the three-dimensional structure of depolarisation canals by combining polarimetric data with stellar parallaxes.
• To investigate whether depolarisation canals trace coherent magnetic field structures aligned with the interstellar medium.
• To test the hypothesis that depolarisation canals mark the edge of the Local Bubble via magnetic field discontinuities.
• To assess the statistical significance of magnetic field orientation changes using Gaia star distances and starlight polarisation.

Proposed method

• Applied rolling Hough transform (RHT) to detect and measure the orientation of depolarisation canals in Faraday depth cubes.
• Cross-correlated canal orientations with HI filament structures from 21 cm line data and plane-of-sky magnetic fields from 353 GHz dust polarisation.
• Used Gaia stellar parallaxes to estimate distances to stars in the line of sight, enabling 3D reconstruction of magnetic field structure.
• Compared observed magnetic field orientations from starlight polarisation with those from depolarisation canals and HI filaments.
• Integrated synthetic HI polarisation fraction (pHI) from velocity-integrated Stokes parameters to assess magnetic field coherence.
• Utilised Faraday tomography to decompose polarised synchrotron emission across Faraday depths from −10 to +15 rad m⁻².

Experimental results

Research questions

• RQ1 Do depolarisation canals in the LOFAR data align with HI filaments and dust-polarised magnetic fields in surrounding fields?
• RQ2 Can stellar parallaxes and polarisation data be used to estimate the distance to depolarisation canals in the 3D ISM?
• RQ3 Is the observed change in magnetic field orientation at ~200 pc consistent with the edge of the Local Bubble?
• RQ4 What is the coherence of the magnetic field in the plane of the sky across the three fields, and how does it correlate with ISM tracers?
• RQ5 How do the Faraday depth structures and polarisation properties in Fields A, B, and C compare to the 3C 196 field and the all-sky average?

Key findings

• Faraday tomography reveals a complex network of diffuse polarised emission between −10 and +15 rad m⁻² across all three fields.
• Depolarisation canals in Fields A and B show strong alignment with both HI filaments and the plane-of-sky magnetic field, indicating a coherent, ordered magnetic field over ~20° scale.
• In Field B, two stellar populations at distances below and above 200 pc exhibit distinct magnetic field orientations that match the depolarisation canal orientations.
• The observed magnetic field change at ~200 pc is consistent with the edge of the Local Bubble, as indicated by 3D ISM maps from starlight extinction data.
• The synthetic HI polarisation fraction (pHI) in Fields A and B exceeds the all-sky average, supporting magnetic field coherence in these regions.
• Field C shows no clear alignment between tracers, lower pHI, and a disordered magnetic field, indicating a lack of large-scale coherence compared to Fields A and B.