The Australia Telescope 20 GHz (AT20G) Survey: analysis of the extragalactic source sample

Abstract

The Australia Telescope 20 GHz (AT20G) survey is a blind survey of the whole Southern sky at 20 GHz with follow‐up observations at 4.8, 8.6 and 20 GHz carried out with the Australia Telescope Compact Array (ATCA) from 2004 to 2008. In this paper we present an analysis of radio spectral properties in total intensity and polarization, sizes, optical identifications and redshifts of the sample of the 5808 extragalactic sources in the survey catalogue of confirmed sources over 6.1 sr in the Southern sky (i.e. the whole Southern sky excluding the strip at Galactic latitude |b| < ). The sample has a flux density limit of 40 mJy. Completeness has been measured as a function of scan region and flux density. Averaging over the whole survey area the follow‐up survey is 78 per cent complete above 50 mJy and 93 per cent complete above 100 mJy. 3332 sources with declination δ < −15° have good quality almost simultaneous observations at 4.8, 8.6 and 20 GHz. The spectral analysis shows that the sample is dominated by flat‐spectrum sources, with 69 per cent having spectral index α208.6 > − 0.5 (S∝να). The fraction of flat‐spectrum sources decreases from 81 per cent for S20 GHz > 500 mJy to 60 per cent for S20 GHz < 100 mJy. There is also a clear spectral steepening at higher frequencies with the median α decreasing from −0.16 between 4.8 and 8.6 GHz to −0.28 between 8.6 and 20 GHz. Simultaneous observations in polarization are available for all the sources at all the frequencies. 768 sources have a good‐quality detection of polarized flux density at 20 GHz; 467 of them were also detected in polarization at 4.8 and/or at 8.6 GHz so that it has been possible to compare the spectral behaviour in total intensity and polarization. We have found that the polarized fraction increases slightly with frequency and decreases with flux density. The spectral indices in total intensity and in polarization are, on average, close to each other, but we also found several sources for which the spectral shape of the polarized emission is substantially different from the spectral shape in total intensity. The correlation between the spectral indices in total intensity and in polarization is weaker for flat‐spectrum sources. Cross‐matches and comparisons have been made with other catalogues at lower radio frequencies, and in the optical, X‐ray and γ‐ray bands. Redshift estimates are available for 825 sources.

High-frequency predictions for number counts and spectral properties of extragalactic radio sources. New evidence of a break at mm wavelengths in spectra of bright blazar sources

“…Because of the incompleteness of the sample for objects with α 20 8 < −0.5, the peak could be more prominent than observed, and it could extend to Δα > ∼ 1. Finally, we note that most inverted-spectrum sources between 5 and 8 GHz (α Our findings agree well with the results of Massardi et al (2011a). A very strong average spectral steepening has also been found in the bright PACO sample (Massardi et al 2011b), where the typical differences between the 5-10 GHz and 30-40 GHz spectral indices are in the range 0.5-1.…”

“…The AT20G survey, thanks to its near-simultaneous observations at 5, 8 and 20 GHz, allows us to study in a quite broad range of frequencies spectral shapes that should be not significantly affected by variability (Massardi et al 2011a). To this aim, we selected the sources of the AT20G-d15S50 sample with S 5 ≥ 200 mJy and with 5-8 GHz spectral index α Δα are 0.13 and 0.23, respectively).…”

“…-The Ryle-Telescope 9C surveys (Taylor et al 2001;Waldram et al 2003 Murphy et al (2010) and in Massardi et al (2011a), and it includes 5890 sources above a flux-density limit of 40 mJy. The completeness of the AT20G catalogue is 93 percent above 100 mJy and 78 percent above 50 mJy in regions south of declination −15…”

THE FIRST FERMI -LAT CATALOG OF SOURCES ABOVE 10 GeV

“…We also added new counterpart catalogs: the Australia Telescope 20-GHz Survey (AT20G) (Murphy et al 86 http://astrogeo.org/vcs/ 2010; Massardi et al 2011) and the Planck Early Release Compact Source Catalog (Planck Collaboration et al 2011).…”

Planckearly results. XV. Spectral energy distributions and radio continuum spectra of northern extragalactic radio sources

“…The Planck-ATCA Co-eval Observations (PACO) project (Massardi et al 2011a) consists of observations of a compilation of sources selected in the Australia Telescope 20 GHz survey (AT20G; Massardi et al 2011b) taken with the Australia Telescope Compact Array (ATCA) in the frequency range 4.5−40 GHz. The observations were carried out at several epochs close in time with Planck satellite observations covering July 2009−August 2010.…”