Deep 20-GHz survey of the Chandra deep field south and SDSS stripe 82: Source catalogue and spectral properties

We present a source catalogue and first results from a deep, bl ind radio survey carried out at 20 GHz with the Australia Telescope Compact Array, with follow-up observations at 5.5, 9 and 18 GHz. The Australia Telescope 20 GHz (AT20G) deep pilot survey covers a total area of 5 deg 2 in the Chandra Deep Field South and in Stripe 82 of the Sloan Digital Sky Survey. We estimate the survey to be 90% complete above 2.5 mJy. Of the 85 sources detected, 55% have steep spectra (α 20.4 −0.5). The steep-spectrum sources tend to have single power-law spectra between 1.4 and 18 GHz, while the spectral indices of the flat- or inverted-sp ectrum sources tend to steepen with frequency. Among the 18 inverted-spectrum (α 20.4 > 0.0) sources, 10 have clearly defined peaks in their spectra with α 5.5 1.4 > 0.15 andα 18 9 <−0.15. On a 3-yr timescale, at least 10 sources varied by more than 15% at 20 GHz, showing that variability is still common at the low flux densities probed by the AT20G-deep pilot survey. We find a strong and puzzling shift in the typical spectral ind ex 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-spec trum population below ∼ 5 mJy. The 5-GHz source-count model by Jackson & Wall (1999), which only includes contributions from FRI and FRII sources, 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.