The Planck-ATCA Coeval Observations (PACO) project: the faint sample

TL;DR

The PACO project provides detailed radio observations of faint sources near the South Ecliptic Pole, revealing spectral steepening, high variability, and extending source counts to lower flux densities, improving understanding of foreground contributions in CMB studies.

Contribution

This study presents the first detailed spectral analysis of a faint radio source sample, extending source counts to lower flux densities and assessing variability and spectral properties.

Findings

Spectral steepening at high frequencies confirmed for faint sources.

Sources exhibit high variability (~40%) over a few years.

Extended source counts by a factor of ~5 at 33 and 40 GHz.

Abstract

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.