Radio surveys with the Australia Telescope Compact Array have been carried out at 1.4 and 2.4 GHz with a limiting flux of ∼0.2 mJy at each frequency in the Marano Field, for which deep optical and X-ray (ROSAT) data are also available. In this paper we present the two radio samples, complete at the 5#x03C3;local level, consisting of 63 and 48 sources at 1.4 and 2.4 GHz, respectively. The 1.4-GHz normalized differential source counts show a flattening below about 1 mJy, in agreement with the results from previous surveys. The 2.4-GHz counts, which are the deepest available at this or similar frequencies (e.g. 2.7 GHz), agree well with the 2.7-GHz counts at higher fluxes and with the extrapolations down to ∼2 mJy based on fluctuation analyses. The 2.4-1.4 GHz spectral index distributions are presented for both of the complete samples in two flux density intervals. The median spectral index for the 1.4-GHz sample remains constant at α∼0.8, down to the lowest fluxes (51.4≃0.2 mJy), while for the higher frequency sample the spectral index distribution flattens in the lower flux density interval (S2.4 < 0.8 mJy). A significant number of sources with inverted spectra (α < 0) do appear in both samples, at a low flux level ( ≾ 2 mJy). These sources, which are about 25 per cent of the complete sample at 2.4 GHz, are probably the ‘bright’ counterpart of the inverted spectrum sources which appear to be almost 50 per cent of the sources in the even deeper radio surveys (at ∼ 20–40 μJy).
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Gruppioni et al. (1997) studied this question.