Optical colours of AGN in the extended Chandra deep field South: obscured black holes in early type galaxies

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

Received: 16 April 2007
Accepted: 23 August 2007

Abstract

We investigate the optical colours of X-ray sources from the Extended Chandra Deep Field South (ECDFS) using photometry from the COMBO-17 survey, aiming to explore AGN - galaxy feedback models. The X-ray sources populate both the “blue” and the “red sequence” on the colour-magnitude diagram. However, sources in the “red sequence” appear systematically more obscured. HST imaging from the “Galaxy Evolution form morphology and SEDs” (GEMS) survey demonstrates that the nucleus does not significantly affect the observed colours, and therefore red sources are early-type systems. In the context of AGN feedback models, this means that there is still remaining material after the initial “blowout”. We argue that this material could not be only left-over from the original merger, and that a secondary cold gas supplier (such as minor interactions or self-gravitational instabilities) must also assist.