Magnetic field evolution in feedback-dominated galaxies
Abstract
Over the last decade, magnetic fields (B-fields) have become a common ingredient in galaxy formation simulations. Although seemingly unimportant in matching observational constraints such as the M*−Mhalo relations, the B-field influences the circumgalactic medium (CGM) gas and determines cosmic ray (CR) propagation, making it an indispensable non-thermal component. Nevertheless, the intricate physics of B-fields, particularly how they evolve to the observed μG-level field, remains largely elusive. In this work, we systematically address the evolution of B-fields over cosmic time using the FIRE-2 (Feedback In Realistic Environments) magnetohydrodynamics (MHD) simulations. We present results for both Milky Way (MW)-mass and dwarf galaxies, with and without CRs. We find that CRs have little impact on B-field properties, while the halo mass controls the saturated B-field strength at z∼ 0. However, all runs reach a halo-independent magnetic-to-turbulent ratio of fB∼ 1−10%, though the methods used to measure turbulence could affect this value. We identify a three-stage evolutionary course for the B-field in tandem with galaxy formation: i. expansion, ii. amplification, and iii. saturation. The feature is universal, though the timings depend on the initial location and star formation history of the gas. Flux-freezing is the predominant driver for the first and last stages, exhibiting power relations B∼ ρ2/3 and ρ1/2, although the latter could also be explained by dynamical and energetic equilibria. The intermediate amplification stage, potentially driven by ``dynamos'', is fuelled by structure formation, feedback, and gas turbulence. We conjecture a numerically triggered, turbulence-driven amplification process to explain this phenomenon.
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BibTeX
@misc{lu2026magnetic,
title = {{Magnetic field evolution in feedback-dominated galaxies}},
author = {Yue Samuel Lu and Dušan Kereš and Philip F. Hopkins and Sam B. Ponnada},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.17216},
eprint = {2610.17216},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.17216},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Magnetic field evolution in feedback-dominated galaxies
AU - Yue Samuel Lu
AU - Dušan Kereš
AU - Philip F. Hopkins
AU - Sam B. Ponnada
PY - 2026
DA - 2026/10/12
PB - arXiv
DO - 10.48550/arxiv.2610.17216
UR - https://arxiv.org/abs/2610.17216
AB - Over the last decade, magnetic fields (B-fields) have become a common ingredient in galaxy formation simulations. Although seemingly unimportant in matching observational constraints such as the M_*−M_halo relations, the B-field influences the circumgalactic medium (CGM) gas and determines cosmic ray (CR) propagation, making it an indispensable non-thermal component. Nevertheless, the intricate physics of B-fields, particularly how they evolve to the observed μG-level field, remains largely elusive. In this work, we systematically address the evolution of B-fields over cosmic time using the FIRE-2 (Feedback In Realistic Environments) magnetohydrodynamics (MHD) simulations. We present results for both Milky Way (MW)-mass and dwarf galaxies, with and without CRs. We find that CRs have little impact on B-field properties, while the halo mass controls the saturated B-field strength at z∼ 0. However, all runs reach a halo-independent magnetic-to-turbulent ratio of f_B∼ 1−10%, though the methods used to measure turbulence could affect this value. We identify a three-stage evolutionary course for the B-field in tandem with galaxy formation: i. expansion, ii. amplification, and iii. saturation. The feature is universal, though the timings depend on the initial location and star formation history of the gas. Flux-freezing is the predominant driver for the first and last stages, exhibiting power relations B∼ ρ^2/3 and ρ^1/2, although the latter could also be explained by dynamical and energetic equilibria. The intermediate amplification stage, potentially driven by ``dynamos'', is fuelled by structure formation, feedback, and gas turbulence. We conjecture a numerically triggered, turbulence-driven amplification process to explain this phenomenon.
ER -