Polarization of bright AT20G sources

For the sake of completeness, we extracted the polarization information for seven extended sources from the 9yr Wilkinson Microwave Anisotropy Probe co-added maps at 23GHz. The full sample of 187 sources constitutes a =~99% complete sample of extragalactic sources brighter than S_20GHz_=500mJy at the selection epoch with declination {delta}<-30{deg}.

The sample has a 91.4% detection rate in polarization at ~20GHz (94 percent if considering the subsample of point-like sources). We have measurements also at 4.8 and 8.6GHz within ~1 month of the 20GHz observations for 172 sources to reconstruct the spectral properties of the sample in total intensity and in polarization: 143 of them have a polarization detection at all three frequencies. We find that there is no statistically significant evidence of a relationship either between the fraction of polarization and frequency or between the fraction of polarization and the total intensity flux density.