SIMBAD references

We present polarization data for 180 extragalactic sources extracted from the Australia Telescope 20GHz (AT20G) survey catalogue and observed with the Australia Telescope Compact Array during a dedicated, high sensitivity run (σP ∼ 1mJy). 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 percent complete sample of extragalactic sources brighter than S20GHz = 500mJy at the selection epoch with declination δ < -30°. The sample has a 91.4 percent 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. This indicates that Faraday depolarization is not very important above 4.8GHz and that the magnetic field is not substantially more ordered in the regions dominating the emission at higher frequencies (up to 20GHz). We estimate the distribution of the polarization fraction and the polarized flux density source counts at ∼ 20GHz.