The Compton-thick AGN in the Chandra Deep Field North

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

Abstract

We present X-ray spectral analysis of the brightest sources ( ) 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 μm data present very high values of the f₂₄/f_R flux ratio suggesting that they are dust obscured galaxies. The low f_x/f_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 . 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.