Chandra and Spitzer observations of CDFS X-ray obscured QSOs^*

¹ Institute of Astronomy & Astrophysics, National Observatory of Athens, Palaia Penteli, 15236, Athens, Greece
² Astrophysics Group, Blackett Laboratory, Imperial College, Prince Consort Road, SW7 2BZ, UK

Received: 27 February 2006
Accepted: 28 October 2006

Abstract

We present Chandra and Spitzer data for the 186, extragalactic, hard 2–10 keV X-ray selected sources, which lie in the central part of the Chandra Deep Field South (CDFS). For the vast majority of sources (99.5%), there is a spectroscopic or photometric redshift available. We classify 17 sources as X-ray obscured QSOs, strictly according to X-ray criteria, i.e. defined as having large hydrogen column densities ( ) and luminosities ( ). The surface density of X-ray obscured QSOs is ~210 . We find 18 candidate Compton-thick   sources, of which three have QSO luminosities ( ). The X-ray obscured QSO comprise a mixed bag of objects, covering the redshift range . Eight of these show narrow-line optical spectra, two show no obscuration in their optical spectra that present broad lines, while for the other seven there is only a photometric redshift available. About half of the X-ray obscured QSOs show high X-ray to optical flux ratios, , and red colours, . Combining the X-ray with the mid-IR or 24  flux can be used as an additional diagnostic to sift out the heavily obscured AGN. All X-ray selected QSOs present red mid-IR colours and can be easily separated among mid-IR sources, demonstrating that mid-IR selection provides a powerful tool for detecting obscured QSOs.