Magnetic fields are present in almost every place in the Universe. Most of the luminous matter is tightly coupled to magnetic fields. Large-scale fields intersperse the gas in the Milky Way, in galaxies and in galaxy clusters. They contribute to the nonlinear interplay of turbulent motions in the interstellar and intracluster medium. The magnetic energy content affects the evolution of galaxies and galaxy clusters, contributes significantly to the total pressure of interstellar gas, is essential for the onset of star formation, and controls the density and distribution of cosmic rays in the interstellar medium (ISM). In spite of their importance, the evolution, structure and origin of magnetic fields are still open problems in fundamental physics and astrophysics.
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. In galaxies, these electrons are probably accelerated in the remnants of supernova explosions, together with other particles of the cosmic ray population. On larger scales, they may be accelerated by shocks, or turbulence, although these are also open questions.
Whatever their origins, magnetic fields are compressed, amplified and distorted by dynamo processes driven by turbulent gas motions, by cosmic rays, and by various feedback mechanisms. The observed synchrotron luminosity allows us to measure the total field strength. Synchrotron radiation is linearly polarized, up to 75% in a fully ordered magnetic field. The polarization plane yields the orientation of the regular field in the plane of the sky. The degree of linear polarization tells us the field's degree of ordering. Moreover, Faraday rotation of the polarization plane provides information on the strength and direction of the field component along the line of sight. Hence, a three-dimensional picture of cosmic magnetic fields can be derived from radio waves.
This figure was derived from combined observations of the prototypical spiral galaxy M51 with the interferometric radio telescope Very Large Array (VLA) near Socorro (New Mexico/USA) and with the 100m single-dish radio telescope near Effelsberg (Germany). The contour lines trace the total radio emission at 4.8 GHz (6 cm wavelength) and the vectors the polarized emission. The vectors indicate that the orientation of the regular magnetic field is spiral and mostly follows the spiral arms, as evident from the background optical image obtained by the Hubble Space Telescope. The magnetic field is strong also between the spiral arms. This was not expected from the idea that the spiral arms are density waves where gas and magnetic fields should be compressed. Instead, the magnetic field must be enhanced between the spiral arms by some other mechanism which is still not understood. Outside of the optical extent of the galaxy, little radio synchrotron emission is detected at high frequencies because the relativistic electrons, responsible for the radio emission and "illuminating" the magnetic field, cannot travel far away from the regions of their origin, supernova remnants. These are embedded into the inner disk of galaxies where the star formation rate is highest. Only in a few cases, radio halos were detected around galaxies seen edge-on.
This figure shows the spiral galaxy NGC 891, seen almost edge-on, which is believed to be very similar to our Milky Way. It was observed at 8.4 GHz (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 observed extent of the radio halo is limited by the large energy losses of the cosmic-ray electrons emitting at this wavelength. At lower frequencies (longer wavelengths) the radio waves are emitted by electrons with lower energies, for which the energy losses are smaller, so that larger radio halos are expected.
In a few spectacular cases the radio synchrotron emission at high frequencies is not restricted to the optical extent. This is always 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. This figure shows the radio map observed at 4.8 GHz (6 cm wavelength) with the Effelsberg 100m telescope. Many more cases of interaction tails and lobes are expected at lower frequencies.