Magnetising the circumgalactic medium of disk galaxies

TL;DR

This study uses high-resolution cosmological simulations to analyze the evolution and structure of magnetic fields in the circumgalactic medium of Milky Way-like galaxies, revealing their origin, amplification, and observational signatures.

Contribution

It demonstrates how galactic outflows and in-situ dynamo processes shape the magnetic field in the CGM, providing new insights into its strength, structure, and observational properties.

Findings

Magnetic fields in the CGM are initially transported by galactic outflows before $z=1$.

An in-situ turbulent dynamo amplifies the magnetic field, saturating before $z=0$.

The magnetic field reaches about 0.1 μG at the virial radius and aligns with observations.

Abstract

The circumgalactic medium (CGM) is one of the frontiers of galaxy formation and intimately connected to the galaxy via accretion of gas on to the galaxy and gaseous outflows from the galaxy. Here we analyse the magnetic field in the CGM of the Milky Way-like galaxies simulated as part of the \textsc{Auriga} project that constitutes a set of high resolution cosmological magnetohydrodynamical zoom simulations. We show that before $z=1$ the CGM becomes magnetised via galactic outflows that transport magnetised gas from the disk into the halo. At this time the magnetisation of the CGM closely follows its metal enrichment. We then show that at low redshift an in-situ turbulent dynamo that operates on a timescale of Gigayears further amplifies the magnetic field in the CGM and saturates before $z=0$. The magnetic field strength reaches a typical value of $0.1\,\mu G$ at the virial radius at $z=0$ and becomes mostly uniform within the virial radius. Its Faraday rotation signal is in excellent agreement with recent observations. For most of its evolution the magnetic field in the CGM is an unordered small scale field. Only strong coherent outflows at low redshift are able to order the magnetic field in parts of the CGM that are directly displaced by these outflows.