  We explore the nature of the radio emission of X-ray selected AGN by
combining deep radio (1.4GHz; 60micro-Jy) and X-ray data with multiwavelength
(optical, mid-infrared) observations in the Extended 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 1Ms 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 (24micron) properties of
these sources are consistent with the infrared/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 radio-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 associated with circumnuclear starbursts are
spherically blocking our view to the central engine.
