Collaboration in the Australia Telescope 20GHz (AT20G) Survey
Our knowledge of the radio-source population at frequencies of tens of GHz is poor, because surveys at these frequencies are very time-consuming. For telescopes with diffraction limited fields of view the number of pointings necessary to cover a given area scales as ν². For a given receiver noise, the time per pointing to reach the flux level S scales as S^-2 so that, for a typical optically thin synchrotron spectrum (S ∝ ν^-0.7), the survey time scales as ν^-3.4: a 20GHz survey takes 240 times longer than a 5 GHz survey to cover the same area to the same flux limit.
On the other hand, Cosmic Microwave Background (CMB) studies, boosted by the on-going NASA WMAP mission and by the forthcoming ESA Planck mission, require an accurate characterization of the high frequency properties of foreground radio-sources, both in total intensity and in polarization. Radio sources are the dominant contaminant of small-scale CMB anisotropies at mm wavelengths: their Poisson contribution to temperature fluctuations is inversely proportional to the angular scale, i.e. linearly proportional to the multipole number l, while the power spectra of the CMB and of Galactic emissions decline at large l. As a result, Poisson fluctuations dominate for l ≥ 400. A high frequency catalogue complete to tens of mJy (the rms noise in Planck channels) over a large area, complemented with a good characterization of source properties, is used to substantially decrease the contaminating effect. Furthermore, forthcoming telescopes in the Southern hemisphere, like the Atacama Large Millimiter Array (ALMA), that will operate in the 90 - 270 GHz range, require suitable calibrators which can be readily selected using large area high frequency surveys. These considerations have motivated the AT20G survey (P.I.: R. Ekers), carried out with the Australia Telescope Compact Array (ATCA) from 2004 to 2007, covering the whole Southern sky at 20 GHz to a typical depth of 50 - 60 mJy. Follow up observations at different frequencies (5, 8, and for a small subset of sources, 93 GHz) , as well as polarization measurements are continuing. The results obtained so far include: source counts, optical identifications, characterization of spectral properties of sources. It was found that: i) sources show complex spectra, sometimes with different shapes in total intensity and in polarization, so that neither the high frequency total intensity nor the polarization behaviour can be reliably estimated from low-frequency information; ii) high frequency steepening is common and correlated with redshift; iii) sources with spectral peaks in the GHz range are common and have high depolarization on the low frequency side of the peak.
People: G. De Zotti, M. Massardi
Collaboration: R. Ekers (P.I.), S. Burke, M. Calabretta, P. Hancock, C. Jackson, M. Kesteven, T. Murphy, K. Newton-McGee, C. Phillips, R. Ricci, E. Sadler, L. Staveley-Smith, M.Walker, J. Wall, W. Wilson
Publications: Massardi et al. (2008), MNRAS, 384,775; Sadler et al. (2008), MNRAS, 385,1656