Abstract. In this paper we present a new sample of 87 Compact Steep Spectrum radio sources (CSS) with flux density ≥0.8 Jy at 0.4 GHz. This sample has been selected from the B3-VLA sample with the aid of new VLA observations at 4.9 and 8.5 GHz which allowed to clean an earlier selection based on VLA 1.5 GHz data. Redshifts, either spectroscopic or photometric, are known for 62% of the sources. About 75% of the sources are resolved or slightly resolved in the new observations. The range of measured linear sizes is from 20 h −1 kpc (selection upper limit) down to ≈0.5 h −1 kpc, which corresponds to our resolution limit. The Largest Linear Size (LLS) distribution is well represented by the power law dN/d(LLS) ∝ LLS −0.6 , consistent with earlier results. The majority of the radio sources smaller than 0.5 h −1 kpc shows a marked spectral flattening at low frequencies. The four frequency spectra computed in the range 0.4-8.5 GHz display a significant steepening at high frequencies in the largest majority of cases. A considerable fraction of source components are polarized, with median values of their fractional polarization ≈6% and ≈4% at 8.5 and 4.9 GHz respectively.

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 flatspectrum 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 S (1.4 GHz) = 80 mJy is 2.2%. 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.

Abstract. This paper is the third of a series, and presents the results of the linear polarisation of the B3-VLA sample, at 1.4, 2.7, 4.8 and 10.5 GHz. We find that flat-spectrum sources are significantly less polarised than the steep ones at 10.5 GHz. A trend is seen for sources with larger linear size to be more strongly polarised. Compact steep-spectrum sources (CSSs) exhibit much stronger depolarisation than non-CSSs. Flat-spectrum sources are characterized by almost constant, and low, degrees of polarisation over the whole wavelength range studied here. We add 143 new Rotation Measures to the available database published by Wielebinski & Krause (1993). An important result is that the decrease of the degree of polarisation is almost linear with wavelength and does not drop to zero at lower frequencies. This behaviour cannot be fitted by any existing model of external depolarisation or intrinsic Faraday dispersion. We consider an intrinsic process producing the observed depolarisation, involving three source components with different emissivities and Faraday depths, while the Faraday rotation must arise from a foreground screen, most likely Galactic in origin.

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