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Radio Observations of the Marano Field and the Faint Radio Galaxy Population

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, in which deep optical and X-ray (ROSAT) data are also available. In this paper we present the two radio samples, complete at the 5 sigma local level, consisting of 63 and 48 sources respectively at 1.4 and 2.4 GHz. The 1.4 GHz normalized differential source counts show a flattening below about one mJy, in agreement with the results from previous surveys. The 2.4 GHz counts, which are the deepest 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 the complete samples in two flux density intervals. The median spectral index for the 1.4 GHz sample remains constant at about 0.8, down to the lowest fluxes (S(1.4)=0.2 mJy), while for the higher frequency sample the spectral index distribution flattens in the lower flux density interval (S(2.4) < 0.8 mJy). A significant number of sources with inverted spectrum (< 0) does appear in both samples, at low flux level (< 2 mJy). These sources, which are about 25% of the complete sample at 2.4 GHz, are probably the ``bright'' counterpart of the inverted spectrum sources which appear to be almost 50% of the sources in the even deeper radio surveys (at about 20-40 microJy).