High-frequency polarization properties of southern Kuhr sources

We have observed 250 of the 258 southern sources in the complete 5 GHz 1 Jy sample by Kuhr et al. (1981) using the Australia Telescope Compact Array (ATCA) at 18.5 GHz. This paper focuses on the polarization properties of this sample, while other properties will be addressed in a future paper. The analysis subdivides the sample into flat and steep spectrum sources using the classification of Stickel et al. (1994), where spectral indices were measured between 2.7 and 5 GHz. The polarized flux has been measured with a S/N>5 for 170 sources (114 flat-spectrum and 56 steep-spectrum). Upper limits have been set for additional 27 sources (12 flat-spectrum and 15 steep-spectrum). The median fractional polarization at 18.5 GHz for the flat-spectrum sub-sample is Pi(18.5)similar or equal to2.7%, which is about a factor of 2 higher than at 1.4 GHz (Pi(1.4)similar or equal to1.4%, based on NVSS data). No evidence was found of significant correlations of the fractional polarization with other source properties. The median value of Pi(18.5) for the steep spectrum sources is similar or equal to4.8%, but our sample might be biased against extended sources. We find some correlation between Pi(18.5) and both the low-frequency (1.4-5 GHz) and the high-frequency (5-18.5 GHz) spectral indices. An important application of this work is in estimating the contamination of CMB polarization maps by extragalactic radio sources. Our results indicate that such contamination is within that estimated by Mesa et al. (2002).