Ab Initio Simulations of a Supernova-driven Galactic Dynamo in an Isolated Disk Galaxy

Abstract

We study the magnetic field evolution of an isolated spiral galaxy, using isolated Milky Way-mass galaxy formation simulations and a novel prescription for magnetohydrodynamic (MHD) supernova feedback. Our main result is that a galactic dynamo can be seeded and driven by supernova explosions, resulting in magnetic fields whose strength and morphology is consistent with observations. In our model, supernovae supply thermal energy, and a low level magnetic field along with their ejecta. The thermal expansion drives turbulence, which serves a dual role by efficiently mixing the magnetic field into the interstellar medium, and amplifying it by means of turbulent dynamo. The computational prescription for MHD supernova feedback has been implemented within the publicly available ENZO code, and is fully described in this paper. This improves upon ENZO's existing modules for hydrodynamic feedback from stars and active galaxies. We find that the field attains µG-levels over Gyr-time scales throughout the disk. The field also develops large-scale structure, which appears to be correlated with the disk's spiral arm density structure. We find that seeding of the galactic dynamo by supernova ejecta predicts a persistent correlation between gas metallicity and magnetic field strength. We also generate all-sky maps of the Faraday rotation measure from the simulation-predicted magnetic field, and present a direct comparison with observations.

Magnetic buoyancy in simulated galactic discs with a realistic circumgalactic medium

“…We note, that the power spectrum on the large scales is fully consistent with a Kazantsev-spectrum (Kazantsev 1968;Kraichnan 1968), known from a small scale turbulent dynamo (Brandenburg & Subramanian 2005;Tobias et al 2011). This is also consistent with the findings of other simulations of isolated disc galaxies (Butsky et al 2017;Rieder & Teyssier 2016, 2017a, as well as those of cosmological zoom-in simulations (Pakmor et al 2017;Rieder & Teyssier 2017b). As this small scale dynamo is one of the central findings of our study, we provide more evidence based on the Kazantsev (1968) theory.…”

“…We find that the structure of the magnetic field strength does not follow exactly the spiral arms in the gas density, but is strongest between two neighbouring spiral arms. This result differs from those reported by other groups (Pakmor & Springel 2013;Butsky et al 2017). The reason for that is that we include a magnetic diffusion term in our simulations, which makes it possible to follow the magnetic field evolution in the non-linear regime.…”

Faraday rotation maps of disc galaxies

“…Thus, our ISM is less turbulent than expected for a fully resolved multiphase ISM. Moreover, the turbulent magnetic field can be increased by local amplification from SN remnants (Butsky et al 2017). In principle noise in the data may also contribute to the difference, but is likely subdominant here.…”

On the origin of magnetic driven winds and the structure of the galactic dynamo in isolated galaxies

“…Recently, there have been various simulations of isolated galaxies, cosmological zoom-in simulations and larger cosmological volumes that include a prescription for solving the equations of magneto hydrodynamics. These simulations provide strong evidence for a small scale turbulent dynamo on scales of galaxies (Beck et al 2012;Pakmor & Springel 2013;Rieder & Teyssier 2016;Butsky et al 2017;Pakmor et al 2017;Rieder & Teyssier 2017;Steinwandel et al 2019) and galaxy clusters (Dolag et al 1999(Dolag et al , 2001Xu et al 2009;Vazza et al 2018;Roh et al 2019). All of these simulations find indications in the magnetic power spectra for small scale turbulence driven amplification of the magnetic field.…”