S317p.84 — Origin of strong magnetic fields in Milky Way-like galactic haloes

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Aug 4th at 6:00 PM until 6:00 PM

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Author(s): Alexander Beck1, Klaus Dolag1, Harald Lesch1

Institution(s): 1. University Observatory Munich

An analytical model predicting the growth rates, the absolute growth times and the saturation values of the magnetic field strength within galactic haloes is presented. The analytical results are compared to cosmological MHD simulations of Milky Way-like galactic halo formation performed with the N-body / SPMHD code GADGET. The halo has a mass of approximately 3*10^{12} solar masses and a virial radius of approximately 270 kpc. The simulations in a LCDM cosmology also include radiative cooling, star formation, supernova feedback and the description of non-ideal MHD. A primordial magnetic seed field ranging from 10^{-10} to 10^{-34} G in strength agglomerates together with the gas within filaments and protohaloes.
There, it is amplified within a couple of hundred million years up to equipartition with the corresponding turbulent energy. The magnetic field strength increases by turbulent small-scale dynamo action. The turbulence is generated by the gravitational collapse and by supernova feedback. Subsequently, a series of halo mergers leads to shock waves and amplification processes magnetizing the surrounding gas within a few billion years. At first, the magnetic energy grows on small scales and then self-organizes to larger scales.
Magnetic field strengths of microG are reached in the center of the halo and drop to nG in the IGM. Analyzing the saturation levels and growth rates, the model is able to describe the process of magnetic amplification notably well and confirms the results of the simulations.
Additionally, we investigate magnetic seed fields created self-consistently by supernova explosions naturally occuring during the star formation in galaxies. Within starforming regions and given typical dimensions and magnetic field strengths in canonical SN remnants, we inject a dipole-shape magnetic field at a rate of nG/Gyr.
In our model for the evolution of galactic magnetic fields, the seed magnetic field determined self-consistently by the star formation process occuring during the formation of cosmic structures. Thus, our model explains the seeding, amplification and distribution of present-day galactic magnetic fields.