Magnetic Fields in Normal Galaxies

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Scientific paper

Interstellar magnetic fields are strongest in massive spiral arms of galaxies (20--30 μG) and in nuclear starburst regions (up to 100 μG). Processes related to star formation tangle the field lines, so that little polarization is observed in star-forming regions. The magnetic energy density in the inner disk of galaxies is larger than the thermal energy density, comparable to that of turbulent gas motions, and is dominant in the outer disk. Large-scale spiral patterns of the regular field are observed in grand-design, flocculent and even some irregular galaxies. In grand-design galaxies the regular fields are aligned parallel to the optical spiral arms, with the strongest regular fields (highest polarization) in interarm regions, sometimes forming magnetic spiral arms between the optical arms. Faraday rotation of the polarization vectors reveals patterns which are signatures of coherent large-scale fields in galactic disks, probably generated by dynamo action. The majority of field structures in galaxies requires a superposition of several dynamo modes. In barred galaxies the magnetic field is mostly aligned with the gas flow, deflected by shear and compressed in the shock. The regular field is already strong in the ``upstream'' region ahead of the shock. Within the circumnuclear ring the magnetic field is strong, with a regular component of spiral shape. Magnetic stress may drive inflow of gas towards the nucleus. -- Present-day radio polarimetry is limited by sensitivity. The next-generation radio telescope, the Square Kilometer Array (SKA), will be able to reveal the full wealth of magnetic structures in galaxies. Cosmic magnetism is one of the Key Science projects for the SKA.

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