Strong magnetic fields and large rotation measures in protogalaxies from supernova seeding

We present a model for the seeding and evolution of magnetic fields in protogalaxies. Supernova (SN) explosions during the assembly of a protogalaxy self-consistently provide magnetic seed fields, which are subsequently amplified by compression, shear flows and random motions. Our model explains the origin of strong magnetic fields of mu G amplitude within the first star-forming protogalactic structures shortly after the first stars have formed. We implement the model into the magnetohydrodynamics (MHD) version of the cosmological N-body/smoothed-particle hydrodynamics (SPH) simulation code gadget and couple the magnetic seeding directly to the underlying multi-phase description of star formation. We perform simulations of Milky-Way-like galactic halo formation using a standard Lambda CDM cosmology and analyse the strength and distribution of the subsequent evolving magnetic field. Within star-forming regions and given typical dimensions and magnetic field strengths in canonical SN remnants, we inject a dipole-shaped magnetic field at a rate of approximate to 10(-9) G Gyr(-1). Subsequently, the magnetic field strength increases exponentially on time-scales of a few tens of millions of years within the innermost regions of the halo. Furthermore, turbulent diffusion, shocks and gas motions transport the magnetic field towards the halo outskirts. At redshift z approximate to 0, the entire galactic halo is magnetized and the field amplitude is of the order of a few mu G in the centre of the halo and approximate to 10(-9) G at the virial radius. Additionally, we analyse the intrinsic rotation measure (RM) of the forming galactic halo over a range of redshift. The mean halo intrinsic RM peaks between redshifts z approximate to 4 and z approximate to 2 and reaches absolute values around 1000 rad m(-2). While the halo virializes towards redshift z approximate to 0, the intrinsic RM values decline to a mean value below 10 rad m(-2). At high redshifts, the distribution of individual star-forming and thus magnetized regions is widespread. This leads to a widespread distribution of large intrinsic RM values. In our model for the evolution of galactic magnetic fields, the seed magnetic field amplitude and distribution are no longer free parameters, but determined self-consistently by the star formation process occurring during the formation of cosmic structures. Thus, this model provides a solution to the seed field problem.