Magnetising the circumgalactic medium of disk galaxies

Ruediger Pakmor, Freeke van de Voort, Rebekka Bieri, Facundo A. Gomez, Robert J. J. Grand, Thomas Guillet, Federico Marinacci, Christoph Pfrommer, Christine M. Simpson, Volker Springel

Published 2019-11-25Version 1

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 at high redshift 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 sets its strength down $z=0$. 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.