Magnetic arms of NGC 6946 traced in Faraday cubes at low radio frequencies

Magnetic fields in galaxies exist on various spatial scales. Large-scale magnetic fields are thought to be generated by the α - Ω dynamo. Small-scale galactic magnetic fields (1 kpc and below) can be generated by tangling the large-scale field. The analysis of field structures with the help of polarized radio continuum emission is hampered by the effect of Faraday dispersion (due to fluctuations in magnetic field and/or thermal electron density), which shifts the signals from large to small scales. At long observation wavelengths, large-scale magnetic fields may become invisible, as in the case of the spectro-polarimetric data cube of the spiral galaxy NGC 6946 observed with the Westerbork Synthesis Radio Telescope in the wavelength range 17-23 cm. The application of rotation measure (RM) synthesis alone does not overcome this problem. We propose to decompose the Faraday data cube into data cubes at different spatial scales by a wavelet transform. Signatures of the "magnetic arms" observed in NGC 6946 at shorter wavelengths then become visible. Our method allows the use of small-scale structures as tracers of large-scale magnetic field patterns in data cubes at long wavelengths, as provided by new-generation radio telescopes.