The properties of extragalactic radio sources selected at 20 GHz

We present some first results on the variability, polarization and general properties of a flux-limited sample of radio sources selected in a blind survey at 20 GHz, the highest frequency at which a sensitive radio survey has been carried out over a large area of sky. Sources with flux densities above 100 mJy in the ATCA 20 GHz Pilot Survey were observed at up to three epochs during 2002-4, including near-simultaneous flux measurements at 4.8, 8.6 and 18 GHz in late 2003. Of the 173 sources in our sample, 65% are identified with candidate QSOs, BL Lac objects or blazars, 20% with galaxies and 15% with faint (b > 22 mag) optical objects or blank fields. On a 1-2 year timescale, the general level of variability at 20 GHz appears to be low. For the 108 sources with good-quality measurements in both 2003 and 2004, the median variability index at 20 GHz over this one-year timescale was 6.9% and only five sources varied by more than 30% in flux density. Most sources in our sample show low levels of linear polarization (typically 1-5%), with a median fractional polarization of 2.3% at 20 GHz. There is a trend for fainter sources to show higher levels of fractional polarization. At least 40% of sources selected at 20GHz show strong spectral curvature over the frequency range 1-20 GHz. We use a radio `two-colour diagram' to characterize the radio spectra of our sample, and confirm that the radio-source population at 20 GHz cannot be reliably predicted by extrapolating the results of surveys at lower frequencies. In particular, direct selection at 20 GHz appears to be a more efficient way of identifying 90 GHz phase calibrators for ALMA than the currently--proposed technique of extrapolation from all-sky surveys at 1-5 GHz.