2014MNRAS.439.1212F

We present a source catalogue and first results from a deep, blind radio survey carried out at 20GHz with the Australia Telescope Compact Array, with follow-up observations at 5.5, 9 and 18GHz. The Australia Telescope 20GHz (AT20G) deep pilot survey covers a total area of 5 deg2 in the Chandra Deep Field South and in Stripe 82 of the Sloan Digital Sky Survey. We estimate the survey to be 90percent complete above 2.5mJy. Of the 85 sources detected, 55percent have steep spectra (α_1.4^20 < -0.5) and 45percent have flat or inverted spectra (α_1.4^20 ≥-0.5). The steep-spectrum sources tend to have single power-law spectra between 1.4 and 18GHz, while the spectral indices of the flat- or inverted-spectrum sources tend to steepen with frequency. Among the 18 inverted-spectrum (α_1.4^20 ≥0.0) sources, 10 have clearly defined peaks in their spectra with α_1.4^5.5 > 0.15 and α_9^18 < -0.15. On a 3-yr time-scale, at least 10 sources varied by more than 15percent at 20GHz, 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 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 ∼ 5mJy, which is followed by a shift back towards a flatter-spectrum population below ∼ 5mJy. The 5-GHz source-count model by Jackson & Wall, 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 ∼ 5mJy. It is therefore possible that another population of sources is contributing to this effect.