Polarization fluctuations due to extragalactic sources

We have derived the relationship between polarization and intensity fluctua tions due to point sources. In the case of a Poisson distribution of a popu lation with uniform evolution properties and constant polarization degree, polarization fluctuations are simply equal to intensity fluctuations times the average polarization degree. Conservative estimates of the polarization degree of the classes of extragalactic sources contributing to fluctuation s in the frequency ranges covered by the forthcoming space missions MAP and Planck Surveyor indicate that extragalactic sources will not be a strong l imiting factor to measurements of the polarization of the Cosmic Microwave Background. (C) 1999 Elsevier Science B.V. All rights reserved.