Faraday tomography of LoTSS-DR2 data

I. Faraday moments in the high-latitude outer Galaxy and revealing Loop III in polarisation^★

¹ Ruđer Bošković Institute, Bijenička cesta 54, 10 000 Zagreb, Croatia
e-mail: aerceg@irb.hr
² Department of Astrophysics/IMAPP, Radboud University, PO Box 9010, 6500 GL Nijmegen, The Netherlands
³ Laboratoire AIM, CEA/CNRS/Université Paris-Saclay, 91191 Gif-sur-Yvette, France
⁴ ASTRON, Netherlands Institute for Radio Astronomy, Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands
⁵ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁶ GEPI & USN, Observatoire de Paris, CNRS, Universié Paris Diderot, 5 place Jules Janssen, 92190 Meudon, France
⁷ Centre for Radio Astronomy Techniques and Technologies, Department of Physics and Electronics, Rhodes University, Grahamstown 6140, South Africa
⁸ School of Natural Sciences, Private Bag 37, University of Tasmania, Hobart TAS 7001, Australia
⁹ Thüringer Landessternwarte, Sternwarte 5, 07778 Tautenburg, Germany
¹⁰ Centre for Astrophysics Research, Department of Physics, Astronomy and Mathematics, University of Hertfordshire, College Lane, Hatfield AL10 9AB, UK

Received: 17 September 2021
Accepted: 22 February 2022

Abstract

Context. Observations of synchrotron emission at low radio frequencies reveal a labyrinth of polarised Galactic structures. However, the explanation for the wealth of structures remains uncertain due to the complex interactions between the interstellar medium and the magnetic field. A multi-tracer approach to the analysis of large sky areas is needed.

Aims. This paper aims to use polarimetric images from the LOFAR Two metre Sky Survey to produce the biggest mosaic of polarised emission in the northern sky at low radio frequencies (150 MHz) to date. The large area this mosaic covers allows for detailed morphological and statistical studies of polarised structures in the high-latitude outer Galaxy, including the well-known Loop III region.

Methods. We produced a 3100 square degree Faraday tomographic cube using a rotation measure synthesis tool. We calculated the statistical moments of Faraday spectra and compared them with data sets at higher frequencies (1.4 GHz) and with a map of a rotation measure derived from extragalactic sources.

Results. The mosaic is dominated by polarised emission connected to Loop III. Additionally, the mosaic reveals an abundance of other morphological structures, mainly narrow and extended depolarisation canals, which are found to be ubiquitous.

Conclusions. We find a correlation between the map of an extragalactic rotation measure and the LoTSS first Faraday moment image. The ratio of the two deviates from a simple model of a Burn slab (Burn, B. J. 1966, MNRAS, 133, 67) along the line of sight, which highlights the high level of complexity in the magnetoionic medium that can be studied at these frequencies.