| Abstract | Depolarization of the polarized radio emission of M 83, due to the turbulent nature of the interstellar medium and magnetic fields, is determined from maps at 2.8,6.3 and 20.3 cm wavelengths. In the inner regions within a galactic radius of ~3.5 arcmin, conspicuous with bright optical spiral arms, the degree of uniformity of the magnetic field is low, while the field in the outer parts of the galaxy is highly uniform. The strength of the total magnetic field, according to equipartition conditions, is 25+/-5 microG near the galaxy's centre, decreasing with a radial scale length of ~7 arcmin. Most of the field in the inner region is turbulent on scales of <= 50 pc, probably due to tangling of field lines anchored in gas clouds associated with active star formation. The unresolved structures in ionized gas and magnetic field give rise to Faraday depolarization at decimeter wavelengths. The observed depolarization in the inner region at all three wavelengths can be modeled by a two-component distribution of thermal plasma: a thick disk of low-density, diffuse gas (central electron density = 0.15+/-0.03 cm^- 3^, filling factor ~0.5) and a thin disk of small H II regions (diameter ~1pc, internal electron density near the galaxy's center = 20+/-4 cm^-3^, filling factor ~0.05). Average Faraday rotation measures are also consistent with (equipartition) field strengths and electron densities from depolarization data. At λ20.3 the inner region of the galaxy is not transparent for polarized emission so that Faraday rotation is no longer proportional to λ^2^. The large-scale variation of Faraday rotation indicates a dynamo driven by gas motions in the bar potential, generating a bisymmetric field. |
