Faraday tomography of LoTSS-DR2 data: I. Faraday moments in the high-latitude outer Galaxy and revealing Loop III in polarisation

Ana Erceg, Vibor Jelić, Marijke Haverkorn, Andrea Bracco, Timothy W. Shimwell, Cyril Tasse, John M. Dickey, Lana Ceraj, Alexander Drabent, Martin J. Hardcastle, Luka Turić

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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.
Original languageEnglish
Article numberA7
Pages (from-to)1-20
Number of pages20
JournalAstronomy & Astrophysics
Early online date1 Jul 2022
Publication statusPublished - 1 Jul 2022


  • astro-ph.GA
  • astro-ph.IM
  • Techniques: interferometric
  • Techniques: polarimetric
  • ISM: general
  • Radio continuum: ISM
  • ISM: structure
  • ISM: magnetic fields


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