The ATESP 5 GHz radio survey - I. Source counts and spectral index properties of the faint radio population

Context. The nature and evolutionary properties of the faint radio population, responsible for the steepening observed in the 1.4 GHz source counts below 1 milliJy, are not yet entirely clear. Radio spectral indices may help to constrain the origin of the radio emission in such faint radio sources and may be fundamental in understanding eventual links to the optical light. Aims. We study the spectral index behaviour of sources that were found in the 1.4 GHz ATESP survey (Prandoni et al. 2000a, A&AS, 146, 31; 2000b, A&AS, 146, 41), considering that the ATESP is one of the most extensive sub-mJy surveys existing at present. Methods. Using the Australia Telescope Compact Array we observed at 5 GHz part of the region covered by the sub-mJy ATESP survey. In particular we imaged a one square degree area for which deep optical imaging in UBVRIJK is available. In this paper we present the 5 GHz survey and source catalogue, we derive the 5 GHz source counts and we discuss the 1.4-5 GHz spectral index properties of the ATESP sources. The analysis of the optical properties of the sample will be the subject of a following paper. Results. The 5 GHz survey has produced a catalogue of 111 radio sources, complete down to a (6 sigma) limit S-lim(5 GHz) similar to 0.4 mJy. We take advantage of the better spatial resolution at 5 GHz (similar to 2 '' compared to similar to 8 '' at 1.4 GHz) to infer radio source structures and sizes. The 5 GHz source counts derived by the present sample are consistent with those reported in the literature, but improve significantly the statistics in the flux range 0.4 less than or similar to S-5 GHz less than or similar to 1 mJy. The ATESP sources show a flattening of the 1.4-5 GHz spectral index with decreasing flux density, which is particularly significant for the 5 GHz selected sample. Such a flattening confirm previous results coming from smaller samples and is consistent with a flattening of the 5 GHz source counts occurring at fluxes less than or similar to 0.5 mJy.