The Planck-ATCA Coeval Observations (PACO) project: the faint sample
  The Planck-ATCA Co-eval Observations (PACO) project collected data between
4.5 and 40 GHz for 482 sources selected within the Australia Telescope 20 GHz
(AT20G) catalogue and observed with the Australia Telescope Compact Array
(ATCA). Observations were done almost simultaneously with the Planck satellite,
in the period between July 2009 and August 2010. In this paper we present and
discuss the data for the complete sample of 159 sources with S(AT20G)>200 mJy
in the Southern ecliptic pole region. The Planck Early Release Compact Source
Catalogue (ERCSC) contains 57 of our sources. A comparison between the PACO
catalogue and the ERCSC confirms that the reliability of the latter is better
than 95 per cent. The missing ERCSC sources are typically associated with the
Large Magellanic Cloud, the Milky Way or are otherwise extended. The spectral
analysis of the PACO faint catalogue shows a spectral steepening of the sources
at high frequencies, confirming the results obtained from the PACO bright
sample. A comparison with AT20G measurements, carried out, on average, a few
years earlier, has demonstrated that, on these timescales, our sources show a
rather high variability with an rms amplitude of ~40 per cent at 20 GHz. The
source spectral properties are found not to vary substantially with flux
density, except for an increase of the fraction of steep spectrum sources at
fainter flux densities. Our data also allow us to extend by a factor ~5
downwards in flux density the source counts at ~33 GHz and ~40 GHz obtained
from Planck's Early Release Compact Source catalogue. This allows us to
substantially improve our control on the contribution of unresolved
extragalactic sources to the power spectrum of small scale fluctuations in CMB
maps.
