Optical colours of AGN in the Extended Chandra Deep Field South: Obscured black holes in early type galaxies

TL;DR

This study examines the optical colours of X-ray detected AGN in the ECDFS to understand obscuration and feedback processes, revealing that red, obscured AGN are hosted by early-type galaxies with residual material.

Contribution

It provides new insights into the nature of obscured AGN in early-type galaxies and suggests secondary cold gas sources contribute to ongoing obscuration.

Findings

Red AGN are more obscured and hosted by early-type galaxies.

Nuclear activity does not significantly alter observed galaxy colours.

Residual material and secondary gas sources sustain obscuration post-merger.

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 GEMS survey demonstrates that the nucleus does not affect significantly 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, but a secondary cold gas supplier (such as minor interactions or self-gravitational instabilities) must also assist.