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Missing magnetic fields in NGC 6946

Submitter: George Heald
Description: Radio continuum observations made with the WSRT (at 18 and 22 cm) have recently been used to study the polarisation of several nearby galaxies. This project has made use of the rotation measure technique that Brentjens & de Bruyn have developed to derive the magnetic fields: it is the first time that this has been done in such a systematic way for a large number of galaxies. The magnetic field traced by these observations gives extra insight into the structure and energetics of the interstellar medium of these galaxies.

In the image above, one of the sample galaxies, NGC 6946, is shown in optical light (images from the DSS), with the magnetic field structure derived from the Westerbork observations overlaid as white lines. The orientations of the magnetic fields have been corrected for Faraday rotation. In this well-studied galaxy (see, for example, the work by Beck 2007, A&A, 470, 539), the magnetic field structure is seen to trace the optical spiral arms very closely. Perhaps the most interesting feature of this image is the lack of magnetic field lines in the bottom right (southwest) quarter of the galaxy. The reason for this oddity is that at the relatively long wavelengths observed in the survey, the emission has become mostly depolarized in that region, possibly as a consequence of the three-dimensional geometry of the magnetic fields.
Copyright: ASTRON
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