Abstract
Faraday tomography allows us to map diffuse polarized synchrotron emission from our Galaxy and use it to interpret the magnetic field in the interstellar medium (ISM). We have applied Faraday tomography to 60 observations from the LOFAR Two-meter Sky Survey (LOTSS) and produced a Faraday depth cube mosaic covering 568 square degrees at high Galactic latitudes, at 4.′3 angular resolution and 1 rad m−2 Faraday depth resolution, with a typical noise level of 50–100 μJy per point spread function (PSF) per rotation measure spread function (RMSF; 40–80 mK RMSF−1). While parts of the images are strongly affected by instrumental polarization, we observed diffuse polarized emission throughout most of the field, with typical brightness between 1 and 6 K RMSF−1, and Faraday depths between − 7 and +25 rad m−2. We observed many new polarization features, some up to 15° in length. These include two regions with very uniformly structured, linear gradients in the Faraday depth; we measured the steepness of these gradients as 2.6 and 13 rad m−2 deg−1. We also observed a relationship between one of the gradients and an H I filament in the local ISM. Other ISM tracers were also checked for correlations with our polarization data and none were found, but very little signal was seen in most tracers in this region. We conclude that the LOTSS data are very well suited for Faraday tomography, and that a full-scale survey with all the LOTSS data has the potential to reveal many new Galactic polarization features and map out diffuse Faraday depth structure across the entire northern hemisphere.
Highlights
Magnetic fields are present throughout interstellar space and play an important role in many aspects of the interstellar medium (ISM), such as cloud collapse during star formation, energy transports and cascades in magnetohydrodynamic turbulence (Beresnyak & Lazarian 2015), and pressure balance between different gas phases (Boulares & Cox 1990).Interstellar magnetic fields can be measured using radio polarization through two processes: synchrotron emission and Faraday rotation
Faraday tomography allows us to map diffuse polarized synchrotron emission from our Galaxy and use it to interpret the magnetic field in the interstellar medium (ISM)
While parts of the images are strongly affected by instrumental polarization, we observed diffuse polarized emission throughout most of the field, with typical brightness between 1 and 6 K rotation measure spread function (RMSF)−1, and Faraday depths between −7 and +25 rad m−2
Summary
Magnetic fields are present throughout interstellar space and play an important role in many aspects of the interstellar medium (ISM), such as cloud collapse during star formation (van Loo et al 2012), energy transports and cascades in magnetohydrodynamic turbulence (Beresnyak & Lazarian 2015), and pressure balance between different gas phases (Boulares & Cox 1990). When applied to 3D image-frequency data this is called Faraday tomography, which produces Faraday depth cubes that map out the diffuse polarized emission as a function of position on the sky and Faraday depth. Such observations can be used to constrain magnetic fields in the ISM and study their properties 3. The HETDEX mosaic The combined Faraday depth cube resulting from the processing described previously covers a total area of sky of 568 square degrees, with a typical noise level between 50 and 100 μJy PSF−1 RMSF−1, with higher noise at the edges and near very bright Stokes I sources. Polarization features with greater variation in Faraday depth than this value should be detected by our observations
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