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Magnetic fields can be determined from the observations of the linear polarization of the radio continuum emission. The non-thermal radio emission, that is dominant at cm-wavelength, originates in the synchrotron emission process, a result of relativistic electrons being braked in magnetic fields when spiralling around the field lines. The emitted E polarization vector is perpendicular to the magnetic field. This linear polarization can suffer Faraday rotation by the passage through the magneto-ionic medium in the observed galaxy or in our own Galaxy. Faraday rotation increases with the square of observation wavelength which can be seen e.g. by comparing the 20cm and 6cm maps of M51. Depolarization can also affect the polarization vectors, especially near areas of rapid change of the polarization angle or of strong Faraday rotation (see M51 at 20cm). The ATLAS OF MAGNETIC FIELDS shows the radio continuum emission of nearby galaxies. Both total intensity and polarized intensity emission is shown. Ideally to trace the magnetic fields a full correction for the Faraday rotation should be applied. When observations have been made at a short radio wavelength (6cm or shorter) the measured E vectors can be rotated through 90 degrees to give the B vector orientation. In cases where full Faraday correction has been applied this is indicated by the title Magnetic field. In most of the galaxies shown in this sample the Faraday rotation is significant at wavelengths of 10cm or longer. The colour scales are arbitrary. Images of total radio intensity are presented in the 'rainbow' colour scale (blue to red, where blue indicates lowest intensities and red indicates highest intensities). Polarized intensities are shown in the 'heat' colour scale (red to white, where red indicates lowest intensities). The frequency of observation, the field size and the angular resolution of the telescope beam are given below each image. Links to original publications are also provided. Reproduction of any of these images requires permission from the MPIfR. FITS data files can be provided on request (for scientific purposes only).
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| MPIfR Home Page Bill Sherwood Modified in November, 2002. |
