Large-scale magnetic-field structure in the spiral galaxy M83

The large-scale structure of magnetic fields in spiral galaxies, as deduced from the linear polarization of scattered starlight and from the intensity and linear polarization of continuum synchrotron radio emission, tends to follow the visible spiral arm structure. Both axisymmetric and bisymmetric patterns are seen, the latter showing a field reversal between the arms. We have measured the linear polarization at a spatial resolution of ∼2 kpc in VLA(Very Large Array) observations of 20-cm continuum radio emission from the bright, nearly face-on, spiral galaxy M83. The strongest linearly polarized emission is from two giant arcs, ∼30 kpc long, situated roughly opposite each other in the dark outer regions of the galaxy, ∼12 kpc from the centre. These regions do not coincide with any prominent spiral arm tracers, and from a comparison with earlier results at 6-cm wavelength, we conclude that the low polarization in the centre of the galaxy indicates disorder in the interstellar magnetic field, probably connected with star formation activity observed there. (author)