Polarization properties of extragalactic radio sources and their contribution to microwave polarization fluctuations

¹ Dipartimento di Astronomia, Università di Padova, Vicolo dell'Osservatorio 2, 35122 Padova, Italy e-mail: mesa@mostro.pd.astro.it
² SISSA, International School for Advanced Studies, Via Beirut 2-4, 34014 Trieste, Italy e-mail: bacci@sissa.it
³ Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Mailstop 50-205, Berkeley, CA 94720, USA
⁴ Osservatorio Astronomico di Padova, INAF, Vicolo dell'Osservatorio 5, 35122 Padova, Italy e-mail: dezotti@pd.astro.it
⁵ IRA/CNR, Via Gobetti 101, 40129 Bologna, Italy
⁶ Dipartimento di Fisica, Università di Bologna, Via Irnerio 46, 40126 Bologna, Italy
⁷ ASTRON/NFRA, Postbus 2, 7990 AA Dwingeloo, The Netherlands e-mail: mack@astron.nl
⁸ Radioastronomisches Institut der Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany

Corresponding author: G. De Zotti, dezotti@pd.astro.it

Received: 5 July 2002
Accepted: 2 September 2002

Abstract

We investigate the statistical properties of the polarized emission of extragalactic radio sources and estimate their contribution to the power spectrum of polarization fluctuations in the microwave region. The basic ingredients of our analysis are the NVSS polarization data, the multifrequency study of polarization properties of the B3-VLA sample (Mack et al. 2002) which has allowed us to quantify Faraday depolarization effects, and the 15 GHz survey by Taylor et al. (2001), which has provided strong constraints on the high-frequency spectral indices of sources. The polarization degree of both steep- and flat-spectrum sources at 1.4 GHz is found to be anti-correlated with the flux density. The median polarization degree at 1.4 GHz of both steep- and flat-spectrum sources brighter than mJy is . The data by Mack et al. (2002) indicate a substantial mean Faraday depolarization at 1.4 GHz for steep spectrum sources, while the depolarization is undetermined for most flat/inverted-spectrum sources. Exploiting this complex of information we have estimated the power spectrum of polarization fluctuations due to extragalactic radio sources at microwave frequencies.
We confirm that extragalactic sources are expected to be the main contaminant of Cosmic Microwave Background (CMB) polarization maps on small angular scales. At frequencies <30 GHz the amplitude of their power spectrum is expected to be comparable to that of the E-mode of the CMB. At higher frequencies, however, the CMB dominates.