2006A&A...457..517P

The nature and evolutionary properties of the faint radio population, responsible for the steepening observed in the 1.4GHz 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. We study the spectral index behaviour of sources that were found in the 1.4GHz ATESP survey (Prandoni et al. 2000A&AS..146...31P; 2000A&AS..146...41P), considering that the ATESP is one of the most extensive sub-mJy surveys existing at present. Using the Australia Telescope Compact Array we observed at 5GHz 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 5GHz survey and source catalogue, we derive the 5GHz source counts and we discuss the 1.4-5GHz spectral index properties of the ATESP sources. The analysis of the optical properties of the sample will be the subject of a following paper. The 5GHz survey has produced a catalogue of 111 radio sources, complete down to a (6σ) limit S_lim(5GHz)∼0.4mJy. We take advantage of the better spatial resolution at 5GHz (∼2" compared to ∼8" at 1.4GHz) to infer radio source structures and sizes. The 5GHz 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≲S_5GHz≲1mJy. The ATESP sources show a flattening of the 1.4-5GHz spectral index with decreasing flux density, which is particularly significant for the 5GHz selected sample. Such a flattening confirm previous results coming from smaller samples and is consistent with a flattening of the 5GHz source counts occurring at fluxes ≲0.5mJy.