Radio observations of the Chandra Deep Field South

Exploring the possible link between radio emission and star formation in X-ray selected AGN

¹ Institute for Astronomy and Astrophysics, National Observatory of Athens, I. Metaxa & V. Pavlou Str., Palaia Penteli, 15236 Athens, Greece e-mail: erovilos@astro.noa.gr
² Astronomical Laboratory, Department of Physics, University of Patras, 26500 Rio-Patras, Greece
³ Astrophysics Group, Imperial College, Prince Consort Rd., London SW7 2AZ, UK
⁴ Centro de Astronomia da Universidade de Lisboa, Observatório Astronómico de Lisboa, 1349-018 Lisboa, Portugal
⁵ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA

Received: 21 December 2006
Accepted: 7 February 2007

Abstract

We explore the nature of the radio emission of X-ray selected AGN by combining deep radio (1.4 GHz; 60 μJy) 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 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 infrared/radio correlation of starbursts. Secondly, most of them are found in galaxies with blue rest-frame optical colours (), suggesting a young stellar population. On the contrary, X-ray/radio matched AGN which are not detected in the mid-infrared have red  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.