Inferring magnetic field strengths in high-redshift lensing galaxies from Faraday rotation measure observations
Abstract
Observations of Faraday rotation in the lensing galaxy of CLASS B1152+199 at z = 0.439 suggest that efficient dynamo mechanisms had already amplified galactic magnetic fields by this epoch. We assess the reliability of this method for deriving magnetic field strengths and the likelihood of observing similarly large Faraday rotation differences. We select 64 star-forming disk galaxies from the TNG50 cosmological magnetohydrodynamical simulation, matched to CLASS B1152+199 in mass and redshift, and forward model them using polarized radiative transfer with POLARIS and subsequent RM synthesis. TNG50 predicts characteristic large-scale field strengths of B0 = 1.9-15.6 μG for individual galaxies, while the combined sample yields B0 = 10.3 μG. Field strengths Bobs inferred from the synthetic observations generally recover B0 within a factor of three. Bobs depends only weakly on the assumed magnetic field geometry, whereas galaxy inclination and beam depolarization have stronger effects. The differential rotation measure recovered from RM synthesis remains correlated with the directly integrated value, although beam depolarization systematically reduces its magnitude. Only 1.41% of directly integrated sightline pairs reproduce the observed ΔRM = 1040 ± 60 rad m−2 of CLASS B1152+199, increasing to 3.16% for sightlines at galactocentric distances comparable to the observed 2.6 and 6.5 kpc. Differential Faraday rotation of gravitationally lensed sources therefore provides a robust probe of large-scale magnetic fields, while the large ΔRM of CLASS B1152+199 is relatively rare among the considered TNG50 analog galaxies.
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BibTeX
@misc{reissl2026inferring,
title = {{Inferring magnetic field strengths in high-redshift lensing galaxies from Faraday rotation measure observations}},
author = {Stefan Reissl and Ralf S. Klessen and Annalisa Pillepich and Sui Ann Mao and Rüdiger Pakmor and Dylan Nelson},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.02328},
eprint = {2610.02328},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.02328},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Inferring magnetic field strengths in high-redshift lensing galaxies from Faraday rotation measure observations
AU - Stefan Reissl
AU - Ralf S. Klessen
AU - Annalisa Pillepich
AU - Sui Ann Mao
AU - Rüdiger Pakmor
AU - Dylan Nelson
PY - 2026
DA - 2026/10/01
PB - arXiv
DO - 10.48550/arxiv.2610.02328
UR - https://arxiv.org/abs/2610.02328
AB - Observations of Faraday rotation in the lensing galaxy of CLASS B1152+199 at z = 0.439 suggest that efficient dynamo mechanisms had already amplified galactic magnetic fields by this epoch. We assess the reliability of this method for deriving magnetic field strengths and the likelihood of observing similarly large Faraday rotation differences. We select 64 star-forming disk galaxies from the TNG50 cosmological magnetohydrodynamical simulation, matched to CLASS B1152+199 in mass and redshift, and forward model them using polarized radiative transfer with POLARIS and subsequent RM synthesis. TNG50 predicts characteristic large-scale field strengths of B_0 = 1.9-15.6 μG for individual galaxies, while the combined sample yields B_0 = 10.3 μG. Field strengths B_obs inferred from the synthetic observations generally recover B_0 within a factor of three. B_obs depends only weakly on the assumed magnetic field geometry, whereas galaxy inclination and beam depolarization have stronger effects. The differential rotation measure recovered from RM synthesis remains correlated with the directly integrated value, although beam depolarization systematically reduces its magnitude. Only 1.41% of directly integrated sightline pairs reproduce the observed ΔRM = 1040 ± 60 rad m^−2 of CLASS B1152+199, increasing to 3.16% for sightlines at galactocentric distances comparable to the observed 2.6 and 6.5 kpc. Differential Faraday rotation of gravitationally lensed sources therefore provides a robust probe of large-scale magnetic fields, while the large ΔRM of CLASS B1152+199 is relatively rare among the considered TNG50 analog galaxies.
ER -