Magnetic fields on FIRE: Comparing B-fields in the multiphase ISM and CGM of simulated L* galaxies to observations

The physics of magnetic fields ($\textbf{B}$) and cosmic rays (CRs) have recently been included in simulations galaxy formation. However, significant uncertainties remain how these components affect evolution. To understand their common observational tracers, we analyze the a set high-resolution, magneto-hydrodynamic, cosmological Milky-Way-like galaxies from FIRE-2 project. We compare mock observables field tracers for with without CRs to observations Zeeman splitting rotation/dispersion measures. find reasonable agreement between both neutral ionised interstellar medium (ISM). that simulated show weaker ISM $|\textbf{B}|$ on average compared magnetic-field-only counterparts. This is manifestation effects diffuse, low density inner circum-galactic (CGM). equipartition ray energy densities may be valid at large ($>$ 1 kpc) scales typical galaxies, but not halos. Within ISM, our follow power-law scaling gas density. extends down hydrogen number $<$ 300 cm$^{-3}$, contrast observationally-derived models, consistent measurements. Finally, generate synthetic rotation measure (RM) profiles projections constraints CGM. While upper limits, improved data are needed detect predicted CGM RMs 10-200 kpc better constrain theoretical predictions.