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

As a pilot study for the first all-sky radio survey at short wavelengths, we have observed 1216 deg(2) 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 -71degrees and -59degrees with an rms noise level of 15 mJy. Follow-up radio imaging of candidate sources above a 4sigma 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 per cent complete at a flux density of 126 mJy and 95 per cent 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| > 5degrees, which can be assumed to be extragalactic. The differential source counts for extragalactic sources in the range 0.1 Jy less than or equal toS(18 GHz)less than or equal to 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 per cent of the extragalactic sources have a flat radio spectrum (alpha(0.843)(18) > -0.5, S(nu)proportional tonu(alpha)), and 29 per cent have inverted radio spectra (alpha(0.843)(18) > 0). The optical identification rate is high: 51 per cent of the extragalactic sources are identified with stellar objects (candidate quasi-stellar objects), 22 per cent with galaxies and only 27 per cent with faint optical objects or blank fields.