X-ray spectral properties of active galactic nuclei in the Chandra Deep Field South

¹ INAF Osservatorio Astronomico di Trieste, via G.B. Tiepolo 11, 34131 Trieste, Italy
² INAF Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
³ Max–Planck–Institut für extraterrestrische Physik, Giessenbachstraße, PF 1312, 85741 Garching, Germany
⁴ Dept. of Physics and Astronomy, The Johns Hopkins University, Baltimore, MD 21218, USA
⁵ Space Telescope Science Institute, 3700 S. Martin Drive, Baltimore, MD 21210, USA
⁶ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street Cambridge, MA 02138
⁷ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany
⁸ Institut d'Astrophysique de Paris, 98bis bd Arago, 75014 Paris France
⁹ Università di Trieste, Dip. Astronomia, via Tiepolo 11, 34131 Trieste, Italy
¹⁰ Center for Astrophysics, University of Science and Technology of China, Hefei, Anhui 230026, PR China

Received: 22 December 2004
Accepted: 7 February 2006

Abstract

We present a detailed X-ray spectral analysis of the sources in the 1Ms catalog of the Chandra Deep Field South (CDFS) taking advantage of optical spectroscopy and photometric redshifts for 321 extragalactic sources out of the total sample of 347 sources. As a default spectral model, we adopt a power law with slope Γ with an intrinsic redshifted absorption N_H, a fixed Galactic absorption and an unresolved Fe emission line. For 82 X-ray bright sources, we are able to perform the X-ray spectral analysis leaving both Γ and N_H free. The weighted mean value for the slope of the power law is , and the distribution of best fit values shows an intrinsic dispersion of . We do not find hints of a correlation between the spectral index Γ and the intrinsic absorption column density N_H.
We then investigate the absorption distribution for the whole sample, deriving the N_H values in faint sources by fixing . We also allow for the presence of a scattered component at soft energies with the same slope of the main power law, and for a pure reflection spectrum typical of Compton-thick AGN. We detect the presence of a scattered soft component in 8 sources; we also identify 14 sources showing a reflection-dominated spectrum. The latter are referred to as Compton-thick AGN candidates.
By correcting for both incompleteness and sampling-volume effects, we recover the intrinsic N_H distribution representative of the whole AGN population, , from the observed one. shows a lognormal shape, peaking around and with . Interestingly, such a distribution shows continuity between the population of Compton-thin and that of Compton-thick AGN.
We find that the fraction of absorbed sources (with  cm^-2) in the sample is constant (at the level of about 75%) or moderately increasing with redshift. Finally, we compare the optical classification to the X-ray spectral properties, confirming that the correspondence of unabsorbed (absorbed) X-ray sources to optical type I (type II) AGN is accurate for at least 80% of the sources with spectral identification (1/3 of the total X-ray sample).