DEPOLARIZATION OF RADIO SYNCHROTRON EMISSION IN SPIRAL GALAXIES

The internal depolarization of linearly polarized radio synchrotron em ission of spiral galaxies by differential Faraday rotation in regular magnetic fields and by Faraday dispersion in random magnetic fields is formulated in one dimension as a function of radio wavelength lambda. 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 a nd have an isotropic distribution of orientation. A graphic representa tion of the calculation procedure is introduced for the Faraday disper sion function. Given a set of typical parameters for spiral galaxies, our model predicts that the fractional polarization is an oscillating function of lambda(2) with minima near zero and decreasing amplitude. Compared with single-size cells, the depolarizing effect of the Kolmog orov-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 li near relation with lambda(2). As a result, Faraday rotation measures a t decimeter wavelengths oscillate in lambda(2) 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 lambda 90 cm, anomalous variation of depolarization with wavelength, excess rotation measures at lambda 20 cm, and the lack of a correlatio n between Faraday rotation measure and depolarization at lambda greate r than or equal to 20 cm.