Extragalactic magnetic fields

Abstract

Magnetic fields were first discovered by the observation of optical polarization in our Galaxy. Theoretical work suggested that magnetic fields can align dust grains and this then can lead to the observed optical polarization. Soon after this discovery the sensitivity of the observations was improved that allowed the detection of optical polarization in nearby galaxies. Radio polarization is a consequence of the synchrotron emission process, the emission of radio waves by relativistic electrons in magnetic fields. The radio waves are emitted with the E vector normal to the magnetic field direction. In the passage through a magneto-ionic medium the position angle of the emitted linear polarization is rotated (the Faraday effect) and hence we can determine the magnetic field along the line of sight. In my contribution I will concentrate on the results of the radio observations of magnetic fields in nearby galaxies. The present knowledge about distant magnetic fields will be briefly sketched. The results on the magnetic fields of the Galaxy are presented by W. Reich (this volume).