Faraday depth similarities across scales: LoTSS and DRAGONS

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Abstract
Faraday rotation of diffuse Galactic synchrotron emission is a powerful tracer of the complex, magnetised interstellar medium (ISM), whose structures span a wide range of spatial scales, requiring both interferometric and single-antenna broadband radio polarisation observations for full characterisation. We compared Faraday rotation in the interferometric LOw-Frequency ARray Two-Metre Sky Survey (LoTSS; 120–168 MHz) and the single-antenna Dominion Radio Astrophysical Observatory Global Magneto-Ionic Medium Survey of the Northern Sky (DRAGONS; 350–1030 MHz), which are complementary in their sensitivity to spatial and Faraday-depth scales. We calculated first moments (M1) of polarised intensity (PI) versus Faraday depth, thus producing PI–weighted mean Faraday depth maps of the regions common to both surveys. These maps show remarkable agreement between the surveys despite the lack of overlap in frequency or spatial-scale coverage. In the northern Galactic region the M1 maps are morphologically similar, with only small spatial shifts in the boundaries between positive and negative M1 and strong pixel-by-pixel correlation. In the southern Galactic region, both surveys trace the Faraday-depth gradient with Galactic longitude previously identified in LoTSS. Faraday depth spectra show consistent numbers and locations of peaks for more than half of the pixels. The strong structural similarity between the surveys, demonstrated by computing structure functions, suggests coupling across spatial scales in the magnetised ISM, thus enabling both interferometric and single-antenna observations to trace the same features. Instances of differences point to ISM configurations where observational effects such as depolarisation dominate or where this coupling breaks down due to local physical conditions.

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