EDP Sciences
Free Access
Volume 507, Number 2, November IV 2009
Page(s) 747 - 756
Section Extragalactic astronomy
DOI https://doi.org/10.1051/0004-6361/200912395
Published online 08 September 2009
A&A 507, 747-756 (2009)
DOI: 10.1051/0004-6361/200912395

The Compton-thick AGN in the Chandra Deep Field North

I. Georgantopoulos1, A. Akylas1, A. Georgakakis1, and M. Rowan-Robinson2

1  Institute of Astronomy & Astrophysics, National Observatory of Athens, Palaia Penteli, 15236 Athens, Greece
    e-mail: ig@astro.noa.gr
2  Astrophysics Group, Blackett Laboratory, Imperial College, Prince Consort Road, SW7 2BZ, UK

Received 27 April 2009 / Accepted 22 August 2009

We present X-ray spectral analysis of the brightest sources ( $f_{2{-}10~{\rm keV}} > 10^{-15}$  $\rm erg\,cm^{-2}\,s^{-1}$) in the Chandra Deep Field North. Our sample consists of 222 sources; for the vast majority (171) either a spectroscopic or a photometric redshift is available. Our goal is to discover the Compton-thick AGN in a direct way i.e. through their X-ray spectra. Compton-thick AGN give away their presence in X-rays either directly through the absorption turnover redshifted in the Chandra passband, or through a flat, reflection-dominated, spectrum. The above selection criteria yield 10 Compton-thick AGN candidates of which the nine are reflection dominated. The IR or sub-mm data where available, corroborate the presence of a heavily obscured nucleus in most cases. All the five candidate Compton-thick sources with available 24 $\mu$m data present very high values of the f24/$f_{\rm R}$ flux ratio suggesting that they are dust obscured galaxies. The low fx/ $f_{\rm IR}$ ratio also suggest the presence of obscured nuclei in many cases. Four of the candidate Compton-thick sources are associated with sub-mm galaxies at high redshifts $z \sim 2$. The number count vs. flux distribution of the candidate Compton-thick AGN as well as their distribution with redshift agree reasonably well with the predictions of the X-ray background synthesis models of Gilli et al.

Key words: X-rays: general -- X-rays: diffuse background -- X-rays: galaxies -- infrared: galaxies

© ESO 2009