Radio observations of the Chandra Deep Field South - Exploring the possible link between radio emission and star formation in X-ray selected AGN

We explore the nature of the radio emission of X-ray selected AGN by combining deep radio (1.4 GHz; 60µJy) and X-ray data with multiwavelength (optical, mid-infrared) observations in the Extend ed Chandra Deep Field South (E-CDFS). The fraction of radio detected X-ray sources increases from 9% in the E-CDFS to 14% in the central region of this field, which has deeper X-ray coverage from the 1 Ms CDFS. We find evidence that the radio emission of up to 60% of the hard X-ray/radio matched AGN is likely associated with star-formation in the host galaxy. Firstly, the mid-IR (24 µm) properties of these sources are consistent with the infra red/radio correlation of starbursts. Secondly, most of them are found in galaxies with blue rest-frame optical colours ( U− V), suggesting a young stellar population. On the contrary, X-ray/radio matched AGN which are not detected in the mid-infrared have red U− V colours suggesting their radio emission is associated with AGN activity. We also find no evidence for a population of heavily obscured r adio-selected AGN that are not detected in X-rays. Finally, we do no confirm previous claims for a correlation between radio emission and X-ray obscuration. Assuming that the radio continuum measures star-formation, this finding is against models where the dust and gas clouds as sociated with circumnuclear starbursts are spherically bl ocking our view to the central engine.