Depolarization of radio synchrotron emission in spiral galaxies

The internal depolarization of linearly polarized radio synchrotron emission of spiral galaxies by differential Faraday rotation in regularmagnetic fields andbyFaradaydispersion in random magnetic fields is formulated in one dimension as a function of radio wavelength λ. The random fields are modeled as a number of cells along the line of sight which obey a Kolmogorov spectrum in size and in field strength and have an isotropic distribution of orientation. A graphic representation of the calculation procedure is introduced for the Faraday dispersion function. Given a set of typical parameters for spiral galaxies, our model predicts that the fractional polarization is an oscillating function of λ2 with minima near zero and decreasing amplitude. Compared with single-size cells, the depolarizing effect of the Kolmogorov-type random fields is much smaller; they only smear the effect of the regular field. However, the random fields have a strong effect on the Faraday polarization angle at long wavelengths and distort its linear relation with λ2. As a result, Faraday rotation measures at decimeter wavelengths oscillate in λ2 so that their sign may reverse without reversals in the regular magnetic field. Our model is able to explain observational phenomena like polarized emission around λ90 cm, anomalous variation of depolarization with wavelength, excess rotation measures at λ20 cm, and the lack of a correlation between Faraday rotation measure and depolarization at λ ≥ 20 cm.