Normalized degree of polarization as a function of wavelength for a two-layer system description of M 51. The measured polarization values for a sector with an azimuth centered at 100° in radial ring 1 (2.4–3.6 kpc) at the three observing wavelengths λλλ 3.5,6.2,20.5 cm are displayed with error bars. All model profiles featured have been constructed from among the following set of magnetic fields: a total regular field strength of 5 μG in the disk and in the halo, an isotropic and anisotropic disk turbulent random field of 10 μG each, and an isotropic and anisotropic halo turbulent random field of 3 μG each. Please consult Table 2 for nomenclature and description of the model types appearing in the legend.