Astron. Astrophys. 321, 71-80 (1997)

Depolarization of radio synchrotron emission in spiral galaxies

X. Chi ^{* 1}, E.C.M. Young ¹ and R. Beck <sup>2

1</sup> Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong
² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany

Received 21 March 1996 / Accepted 23 September 1996

Abstract

The internal depolarization of linearly polarized radio synchrotron emission 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 . 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 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 . As a result, Faraday rotation measures at decimeter wavelengths oscillate in 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 90 cm, anomalous variation of depolarization with wavelength, excess rotation measures at 20 cm, and the lack of a correlation between Faraday rotation measure and depolarization at  cm.

Key words: polarization techniques: polarimetric radio continuum: galaxies galaxies: spiral galaxies: magnetic fields galaxies: ISM

* Now at Dept. of Computer Science, Vanderbilt University, Nashville TN37235, USA

Send offprint requests to: R. Beck: rbeck@mpifr-bonn.mpg.de

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Contents

• 1. Introduction
• 2. The formulation
  • 2.1. General remarks
  • 2.2. Depolarization in single-size cells with random magnetic fields
  • 2.3. Depolarization in random and regular fields
  • 2.4. Depolarization in a Kolmogorov-type random field
  • 2.5. Faraday rotation measures
• 3. Discussion and conclusions
• Appendix A
• Appendix A: probability distribution of the product of two random variables
• Acknowledgements
• References

© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998
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