Galaxies in southern bright star fields I. Near-infrared imaging

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.