A subgrid turbulent mean-field dynamo model for cosmological galaxy formation simulations

Magnetic fields have been included in cosmological simulations of galaxy formation only recently. In this paper, we develop a new subgrid model for the turbulent dynamo that takes place in the supersonic interstellar medium in star-forming galaxies. It is based on a mean-field approach that computes the turbulent kinetic energy at unresolved scales and modifies the induction equation to account for the corresponding αdynamo. Our subgrid model depends on one free parameter, the quenching parameter, that controls the saturation of the subgrid dynamo. Thanks to this mean-field approach, we can now model the fast amplification of the magnetic field inside turbulent star-forming galaxies without using prohibitively expensive high-resolution simulations. We show that the evolution of the magnetic field in our zoom-in Milky Way-like galaxy is consistent with a simple picture, in which the field is in equipartition with the turbulent kinetic energy inside the star-forming disc, with a field strength around 10 μG at low redshift, while at the same time strong galactic outflows fill the halo with a slightly weaker magnetic field, whose strength (10 nG) is consistent with the ideal magnetohydrodynamic dilution factor. Our results are in good agreement with recent theoretical and numerical predictions. We also compare our simulation with Faraday depth observations at both low and high redshifts, seeing o v erall good agreement with some caveats. Our model naturally predicts stronger magnetic fields at high redshift (around 100 μG in the galaxy and 1 μG in the halo), but also stronger depolarization effects due to stronger turbulence at early time.