1994A&A...292..409B

Radio continuum observations of the spiral galaxy NGC 253 with the Effelsberg and VLA telescopes reveal polarized emission from the bar and halo regions. Within the bar Faraday depolarization is strong at 1.5 and 5GHz, due to ionized gas with n_e≃0.1-3cm^–3 which is mixed with turbulent magnetic fields of ≃17µG estimated strength. Even at 10GHz the degree of polarization in the bar is low (only ≃5% east and ≃2% west of the nucleus) due to beam depolarization by unresolved tangled fields. In contrast, the magnetic fields in the halo are highly uniform, as indicated by fractional polarizations up to 40% at 10 GHz. Faraday depolarization in the halo at 1.5 GHz calls for a warm, clumpy gas component with n_e≃0.02cm^–3 and ≃6µG turbulent fields. We detected Faraday rotation in the bar, with rotation measures |RM|≃100rad/m² (between 10 and 5GHz) having different signs east and west of the nucleus. Below 5 GHz Faraday rotation is strongly reduced by the limited transparency for polarized emission in the bar. Faraday rotation in the halo in two regions at ∼5kpc above and below the plane with RM≃-7rad/m² between 10 and 1.5GHz can be ascribed to hot gas with <n_e≥∼0.002cm^–3 and uniform fields along the line of sight of <B_u||≥~-2µG. The magnetic field structure in the bar and halo of NGC 253 is best described by the quadrupole-type dynamo mode S0, with a ring-like field in the bar and a field mainly parallel to the plane in a co-rotating halo. A major perturbation occurs in the east where the field is perpendicular to the plane and follows a "spur". The galactic wind is suppressed by the dominating plane-parallel field, except along the spur.