Faraday tomography of LoTSS-DR2 data: III. Revealing the Local Bubble and the complex of local interstellar clouds in the high-latitude inner Galaxy

TL;DR

This study uses LOFAR data and Faraday tomography to map the local interstellar medium, revealing the structure of the Local Bubble and nearby interstellar clouds in high-latitude regions.

Contribution

It introduces a new mosaic of LoTSS-DR2 data with Faraday depth analysis, linking magnetic structures to the Local Bubble and local interstellar clouds.

Findings

Ordered Faraday structures align with HI filaments and magnetic fields.

Estimated distances place structures near the Local Bubble wall (40-80 pc).

Detected a large Faraday depth gradient associated with the Local Bubble wall.

Abstract

In this work, we present a new mosaic created with the second release of LOFAR Two-Metre Sky Survey data (LoTSS-DR2), which probes polarised synchrotron emission in the high-latitude inner Galaxy. Our objective is to characterise the observed emission through multi-tracer analysis to better understand the volume and the structures that may be observed with LOFAR. Furthermore, we exploit Faraday depth as a unique tool to probe the diffuse magnetised structure in the local ISM. We produced a mosaic Faraday cube of LoTSS-DR2 data by applying a rotation measure synthesis algorithm. From the cube, we constructed Faraday moment maps to characterise the nature of spectra. Additionally, we quantified the linear depolarisation canals using the Rolling Hough transform and used them to search for alignment with other data sets. Utilising LoTSS-DR2 observations alongside complementary data sets including Planck polarisation data, HI emission maps, and starlight polarisation measurements, we estimated the distance to the Faraday structures. The Faraday cube reveals an ordered structure across two-thirds of the observed area, whose orientation aligns well with that of both the HI filaments and the magnetic field. We estimate the minimum distance to the Faraday structures to be between 40 and 80 pc, which puts them in the vicinity of the Local Bubble wall. The emission is organised in a large gradient in Faraday depth whose origin we associate with the curved wall of the Local Bubble. Comparing our data with a model of the Local Bubble wall, we conclude that we might be probing a contribution of the medium inside the Local Bubble cavity as well, corresponding to the complex of local interstellar clouds. Moreover, we propose a toy model incorporating an ionised front of finite thickness into the Local Bubble wall, as a curved, cold neutral shell alone is insufficient to produce the observed gradient.