Magnetic Fields on FIRE: Comparing B-fields in the multiphase ISM and CGM of Simulated L${}_{\ast }$ Galaxies to Observations

Abstract

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

15 pages, 9 figures. Accepted in MNRAS 2022 August 26. Received 2022 August 25; in original form 2022 June 10

References in corpus (16)

• Revised equipartition & minimum energy formula for magnetic field strength estimates from radio synchrotron observations
• The magnetic field and turbulence of the cosmic web measured using a brilliant fast radio burst
• Magnetohydrodynamic Simulations of Disk Galaxy Formation: the Magnetization of The Cold and Warm Medium
• Low-frequency Faraday rotation measures towards pulsars using LOFAR: probing the 3-D Galactic halo magnetic field
• 3-D gas-phase elemental abundances across the formation histories of Milky Way-mass galaxies in the FIRE simulations: initial conditions for chemical tagging
• Cosmic rays and non-thermal emission in simulated galaxies: III. probing cosmic ray calorimetry with radio spectra and the FIR-radio correlation
• Simulating radio synchrotron emission in star-forming galaxies: small-scale magnetic dynamo and the origin of the far infrared-radio correlation
• Evolution of giant molecular clouds across cosmic time
• A small-scale dynamo in feedback-dominated galaxies - III. Cosmological simulations
• Magnetic fields in the Milky Way from pulsar observations: effect of the correlation between thermal electrons and magnetic fields
• Constraining Magnetic Fields in the Circumgalactic Medium
• Extragalactic Magnetism with SOFIA (Legacy Program) -- I: The magnetic field in the multi-phase interstellar medium of M51
• The impact of cosmic rays on dynamical balance and disk-halo interaction in Lstar disk galaxies
• Stellar feedback strongly alters the amplification and morphology of galactic magnetic fields
• A dynamo amplifies the magnetic field of a Milky-Way-like galaxy
• Rotation measure and synchrotron emission signatures in simulations of magnetized galactic discs

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• Deep Synoptic Array Science: Implications of Faraday Rotation Measures of Localized Fast Radio Bursts
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