First results from the Australia Telescope Compact Array 18-GHz pilot survey

Abstract

As a pilot study for the first all‐sky radio survey at short wavelengths, we have observed 1216 deg² of the southern sky at 18 GHz (16 mm) using a novel wide‐band (3.4‐GHz bandwidth) analogue correlator on one baseline of the Australia Telescope Compact Array. We scanned a region of sky between declination −71° and −59° with an rms noise level of 15 mJy. Follow‐up radio imaging of candidate sources above a 4σ detection limit of 60 mJy resulted in 221 confirmed detections, for which we have measured accurate positions and flux densities. For extragalactic sources, the survey is roughly 70% complete at a flux density of 126 mJy and 95% complete above 300 mJy. Almost half the detected sources lie within a few degrees of the Galactic plane, but there are 123 sources with |b| > 5°, which can be assumed to be extragalactic. The differential source counts for extragalactic sources in the range 0.1 Jy ≤S_{18 GHz} ≤ 3 Jy are well fitted by a relation of the form n(S) = 57 (S/Jy)^{−2.2 ± 0.2} Jy⁻¹ sr⁻¹, in good agreement with the 15‐GHz counts published by Taylor et al. and Waldram et al. Over 70% of the extragalactic sources have a flat radio spectrum (α¹⁸_0.843 > −0.5, S_ν∝ν^α) and 29% have inverted radio spectra (α¹⁸_0.843 > 0). The optical identification rate is high: 51% of the extragalactic sources are identified with stellar objects (candidate quasi‐stellar objects), 22% with galaxies and only 27% with faint optical objects or blank fields.