Galactic magnetic fields, cosmic rays and winds

Michał Hanasz

This lecture reviews recent investigations of the dynamical eects of cosmic rays (CRs) in the interstellar medium of disk galaxies. We focus on the role of CRs for generation of galactic magnetic fields as well as galactic winds. Recent simulation models of magnetised interstellar medium, including the CR gas described by the diusion-advection equation, indicate that cosmic rays accelerated in supernova remnants play a signicant role as drivers of magnetic eld amplication in galaxies. It has been demonstrated that weak dipolar magnetic elds supplied on small SNremnant scales can be amplied exponentially, by the CR-driven dynamo, to the present equipartition values, and transformed simultaneously to large galacticscales. The resulting magnetic field structure in an evolved galaxy appears spiral in the face-on view and reveals a so called X-shaped structure in the edge-on view. Those X-shaped structures in synchrotron radio maps result from the advection of disk magnetic fields by galactic winds. Driven by the additional pressure gradient of the CR relativistic fluid, the wind speed can exceed the escape velocity of the galaxy. The global mass loading, i.e. the ratio of the mass of gas leaving the galactic disk in the CR-driven wind to the star formation rate, becomes of order unity. These winds help in explaining the low eciencies for the conversion of gas into stars in galaxies, as well as the early enrichment of the intergalactic medium with metals.

Proceedings of the Polish Astronomical Society, vol. 4, 141-156 (2017)

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