Faraday Complexity and Depolarization in LOFAR Two-metre Sky Survey (LoTSS-DR2) Polarized Radio Sources

TL;DR

This study analyzes the polarization properties of 1,565 radio sources from LoTSS-DR2 across 120-168 MHz, revealing prevalent Faraday complexity and environmental turbulence affecting depolarization.

Contribution

It provides the first broadband spectro-polarimetric analysis of a large LOFAR sample, modeling Faraday depolarization with multiple components and examining environmental effects.

Findings

43.2% of sources require multiple Faraday components.

External Faraday dispersion dominates depolarization behavior.

Faraday dispersion correlates with redshift, indicating evolving environments.

Abstract

We present a broadband spectro-polarimetric analysis of 1,565 polarized radio sources from the LOFAR Two-Metre Sky Survey Data Release 2 (LoTSS-DR2) RM Grid catalogue. This study uses frequency-dependent Stokes Q and U spectra across the 120-168 MHz LOFAR HBA band to investigate their polarization properties. The polarization behaviour of each source is modelled with multi-component Faraday depolarization models to investigate the magneto-ionic environments responsible for low-frequency depolarization. Significant Faraday complexity is observed throughout the sample, with 43.2% of sources requiring two or three Faraday components. External Faraday dispersion dominates the depolarization behaviour, with 54.1% of sources classified as external-screen dominated and approximately 60% showing statistically significant evidence for turbulent external Faraday-active media, while only 10.3% are consistent with pure internal differential Faraday rotation. The intrinsic polarization angle and RM separations between fitted components are generally small, suggesting that many RM components trace physically related emission regions embedded within common magneto-ionic environments. A weak but statistically significant anti-correlation is detected between the polarization spectral index, $\beta$, and weighted Faraday dispersion, $\sigma_{\rm RM,wtd}$, for two-component systems, whereas one- and three-component populations show no significant trend. The rest-frame Faraday dispersion, $\sigma_{\rm RM,rest}$, exhibits significant positive correlations with redshift for the external-screen dominated and mixed depolarization populations, even after controlling for radio luminosity, indicating increasingly turbulent or strongly magnetized environments surrounding radio AGN at earlier cosmic epochs.