CORRELATION OF MAGNETIC AND OPTICAL STRUCTURE IN THE BARRED SPIRAL GALAXY M83

THE barred spiral (SBc) galaxy M83 was previously studied thoroughly in the optical regime 1-3, and is known to possess an ordered magnetic field, with radio emission showing linear polarization of up to 60% in the outer regions 4-6. Earlier radio studies were done at lower frequencies, where Faraday rotation of the polarization by the magnetic field is important, but we have now observed M83 using the 100-m Effelsberg telescope at a frequency of 10.6 GHz, at which Faraday rotation should be less than 1-degrees. There is significant polarization in the inner part of the galaxy, where we find that the magnetic field closely follows the optical structure. We see some deviations from the ordered magnetic structure that coincide with optical patchiness, and two pronounced polarization minima that correspond with enhancements of CO emission 7. Our results suggest that the inner structure of the galaxy is more orderly than earlier work 6 had indicated, and that any gross disorder, perhaps due to star formation, occurs only in the innermost regions. In addition, the observed distribution of Faraday rotation across the galaxy, deduced from comparison of observations at different frequencies, provides evidence that a significant component of the magnetic field extends away from the disk.