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Synchrotron emission is one of few observable tracers of galactic magneticfields (\textbf{B}) and cosmic rays (CRs). Much of our understanding of\textbf{B} in galaxies comes from utilizing synchrotron observations inconjunction 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 selfconsistently evolve CR proton, electron, and positron spectra from MeV to TeVenergies, we present the first synthetic synchrotron emission predictions fromsimulated L$_{*}$ galaxies with "live" spectrally-resolved CR-MHD. We find thatsynchrotron emission can be dominated by relatively cool and dense gas,resulting in equipartition estimates of \textbf{B} with fiducial assumptionsunderestimating the "true" \textbf{B} in the gas that contributes the mostemission by factors of 2-3 due to small volume filling factors. Motivated byour results, we present an analytic framework that expands upon equipartitionmodels for estimating \textbf{B} in a multi-phase medium. Comparing ourspectrally-resolved synchrotron predictions to simpler spectral assumptionsused in galaxy simulations with CRs, we find that spectral evolution can becrucial for accurate synchrotron calculations towards galactic centers, whereloss terms are large.

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