A Deep 20-GHz Survey in the South

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Clik here to view.© Tom Franzen

A team of astronomers, led by Tom Franzen (CSIRO) and including ASTRON astronomer Elizabeth Mahony, have recently presented the source catalogue and first results from a deep, blind radio survey carried out at 20 GHz with the Australia Telescope Compact Array. The Australia Telescope 20 GHz (AT20G) deep pilot survey covers a total area of 5 square degrees in the Chandra Deep Field South and in Stripe 82 of the Sloan Digital Sky Survey and was observed at 5, 9 and 20 GHz down to a flux density limit of 2.5 mJy.

One of the interesting results found in this study is that there is a strong and puzzling shift in the typical spectral index of the 15-20 GHz source population when combining data from the AT20G, Ninth Cambridge and Tenth Cambridge surveys: there is a shift towards a steeper-spectrum population when going from 1 Jy to 5 mJy, which is followed by a shift back towards a flatter-spectrum population below 5 mJy. Comparing with source count models of high-frequency radio sources (which include contributions from FRI and FRII radio galaxies and star-forming galaxies) does not reproduce the observed flattening of the flat-spectrum counts below 5 mJy. It is therefore possible that another population of sources is contributing to this effect. A link to the published paper can be found here: http://adsabs.harvard.edu/abs/2014MNRAS.439.1212F

Image: On the left is shown the 20 GHz map of the 03 hr field produced by combining approximately 3500 individual maps (top panel) and a zoomed in region (bottom panel). On the right are plots showing how the fraction of steep spectrum sources changes as a function of flux density. Two different cutoffs in spectral index are used: the percentage of sources steeper than -0.8 is shown in the top panel and the percentage of sources steeper than -0.5 shown in the bottom panel.