Synchrotron Emission on FIRE: Equipartition Estimators of Magnetic Fields in Simulated Galaxies with Spectrally-Resolved Cosmic Rays

TL;DR

This paper uses FIRE simulations with spectrally-resolved cosmic rays to evaluate how well equipartition assumptions estimate galactic magnetic fields from synchrotron emission, revealing underestimations and the importance of spectral evolution.

Contribution

It presents the first synthetic synchrotron emission predictions from self-consistent, spectrally-resolved CR-MHD simulations of galaxies, and develops an analytic framework to improve magnetic field estimates.

Findings

Equipartition estimates underestimate true magnetic fields by factors of 2-3.

Synchrotron emission is dominated by cool, dense gas with small volume filling factors.

Spectral evolution significantly affects synchrotron calculations in galactic centers.

Abstract

Synchrotron emission is one of few observable tracers of galactic magnetic fields (\textbf{B}) and cosmic rays (CRs). Much of our understanding of \textbf{B} in galaxies comes from utilizing synchrotron observations in conjunction with several simplifying assumptions of equipartition models, however it remains unclear how well these assumptions hold, and what \textbf{B} these estimates physically represent. Using FIRE simulations which self consistently evolve CR proton, electron, and positron spectra from MeV to TeV energies, we present the first synthetic synchrotron emission predictions from simulated L$_{*}$ galaxies with "live" spectrally-resolved CR-MHD. We find that synchrotron emission can be dominated by relatively cool and dense gas, resulting in equipartition estimates of \textbf{B} with fiducial assumptions underestimating the "true" \textbf{B} in the gas that contributes the most emission by factors of 2-3 due to small volume filling factors. Motivated by our results, we present an analytic framework that expands upon equipartition models for estimating \textbf{B} in a multi-phase medium. Comparing our spectrally-resolved synchrotron predictions to simpler spectral assumptions used in galaxy simulations with CRs, we find that spectral evolution can be crucial for accurate synchrotron calculations towards galactic centers, where loss terms are large.