Most of what we know about astrophysical magnetic fields has been detected via radio astronomical observations. Most of the cosmic broad-band ("continuum") radio emission is synchrotron radiation by relativistic electrons which spiral around magnetic field lines. These electrons are probably accelerated in the remnants of supernova explosions, together with other particles of the cosmic ray population. The observed synchrotron luminosity allows us to measure the total field Strength. Synchrotron radiation is linearly polarised, up to 75% in a fully regular magnetic field. The polarisation plane yields the orientation of the regular field in the plane of the sky. The degree of linear polarisation tells us the field's degree of ordering. Moreover, Faraday rotation of the polarisation plane provides information on the field component along the line of sight. Hence, a three-dimensional picture of cosmic magnetic fields can be derived from radio waves.
Figure 2 shows the galaxy NGC 891 which is believed to be very similar to our Milky Way. It was observed at 3.6 cm wavelength with the Effelsberg 100m telescope. The background optical image is from the CFHT Observatory. The X-shaped structure of the magnetic fields indicates the action of a galactic wind.
The few spectacular cases where the radio synchrotron emission at high frequencies is not restricted to the optical extent are all related to interactions between galaxies. For example, huge radio lobes were discovered on two sides of the galaxy NGC 4569 which is located in the dense Virgo cluster of galaxies where interactions are frequent. Figure 3 shows the radio map observed at 6 cm wavelength with the Effelsberg telescope.