Galactic magnetic fields

"The typical average equipartition strength for spiral galaxies is about 10 μG (microGauss) or 1 nT (nanoTesla). For comparison, the Earth's magnetic field has an average strength of about 0.3 G (Gauss or 30 μT (microTesla)). Radio-faint galaxies like M 31 and M 33, our Milky Way's neighbors, have weaker fields (about 5 μG), while gas-rich galaxies with high star-formation rates, like M 51, M 83 and NGC 6946, have 15 μG on average. In prominent spiral arms the field strength can be up to 25 μG, in regions where also cold gas and dust are concentrated. The strongest total equipartition fields (50-100 μG) were found in starburst galaxies, for example in M 82 and the Antennae, and in nuclear starburst regions, for example in the centers of NGC 1097 and of other barred galaxies.

Galactic fields are sufficiently strong to be dynamically important: they drive mass inflow into the centers of galaxies, they modify the formation of spiral arms and they can affect the rotation of gas in the outer regions of galaxies. Magnetic fields provide the transport of angular momentum required for the collapse of gas clouds and hence the formation of new stars.

The degree of radio polarization within the spiral arms is only a few %; the field in the spiral arms must be mostly tangled. The ordered (regular or anisotropic) fields traced by polarized synchrotron emission are generally strongest (10-15 μG) in the regions between the optical spiral arms. This can possibly be explained by a dynamo wave which is phase shifted with respect to the density wave producing the spiral arms."