Magnetic Field Structure from Synchrotron Polarization

Abstract


Total magnetic fields in spiral galaxies, as observed through their total synchrotronemission, are strongest (up to ≲30 µG) in the spiral arms. The degree of radio polarization is low; the field in the arms must be mostly turbulent or tangled. Polarized synchrotronemission shows that the resolved regular fields are generally strongest in the interarm regions (up to ≲15 µG), sometimes forming “magnetic arms” parallel to the optical arms. The field structure is spiral in almost every galaxy, even in flocculent and bright irregular types which lack spiralarms. The observed large-scale patterns of Faraday rotation in several massive spiral galaxies reveal coherent regular fields, as predicted by dynamo models. However, in most galaxies observed so far no simple patterns of Faraday rotation could be found. Either many dynamo modes are superimposed and cannot be resolved by present-day telescopes, or most of the apparently regular field is in fact anisotropic random, with frequent reversals, due to shearing and compressing gas flows. In galaxies with massive bars, the polarization pattern follows the gas flow. However, around strong shocks in bars, the compression of theregular field is much lower than that of the gas; the regular field decouples from the cold gas and is strong enough to affect the flow of the diffuse warm gas. – The average strength of the total magnetic field in the Milky Way is 6µG near the sun and increases to 20–40 μG in the Galactic center region.The Galactic field is mostly parallel to the plane, except in the center region. Rotation measure data from pulsars indicate several field reversals, unlike external galaxies, but some reversals could be due to distortions of thenearby field.