A Polarisation Survey of Bright Extragalactic AT20G Sources
  We present polarisation data for 180 extragalactic sources extracted from the
Australia Telescope 20 GHz (AT20G) survey catalog, and observed with the
Australia Telescope Compact Array during a dedicated, high sensitivity run. For
the sake of completeness we extracted the polarisation information for 7
extended sources from the 9-yr WMAP coadded maps at 23 GHz. The full sample of
187 sources constitutes a 99% complete sample of extragalactic sources brighter
than S(20 GHz)=500mJy at the selection epoch with declination <-30 deg. The
sample has a 91.4% detection rate in polarisation at 20GHz (94% if considering
the sub-sample of point like sources). We have measurements also at 4.8 and 8.6
GHz within 1 month of the 20GHz observations for 172 sources to reconstruct the
spectral properties of the sample in total intensity and in polarisation: 143
of them have a polarisation detection at all three frequencies. We find that
there is no statistically significant evidence of a relationship either between
the fraction of polarisation and frequency or between the fraction of
polarisation and the total intensity flux density. This indicates that Faraday
depolarisation is not very important above 4.8 GHz and that the magnetic field
is not substantially more ordered in the regions dominating the emission at
higher frequencies (up to 20 GHz). We estimate the distribution of the
polarisation fraction and the polarised flux density source counts at 20GHz.
