The nature of the faint radio source population from observations of the Subaru/XMM-Newton Deep Field

We briefly describe the radio image of the Subaru/XMM-Newton Deep Field. Using deep optical and X-ray imaging data, we investigate how the properties of extragalactic radio sources vary with flux density. We find that the optical colours of our faintest (0.1 mJy < S(1.4) < 0.3 mJy) radio sources are indistinguishable from those at brighter flux densities, but a significant fraction of these objects are bright in X-rays. The optical colours and morphologies and X-ray fluxes of these sources indicate that they are radio-quiet QSOs, and further interpretation leads us to conclude that as many as half of the new population of sources which causes the flattening of the radio source counts at S(1.5) less than or similar to 0.5 mJy are radio-quiet AGN, rather than starburst galaxies. We suggest that studying radio sources with S(1.4) similar to 0.1 mJy could help to understand the make-up of the Cosmic X-ray Background.