The high-redshift (z > 3) active galactic nucleus population in the 4-Ms Chandra Deep Field-South

We present results from a spectral analysis of a sample of high-redshift (z > 3) X-ray-selected active galactic nucleus (AGN) in the 4-Ms Chandra Deep Field-South (CDF-S), the deepest X-ray survey to date. The sample is selected using the most recent spectroscopic and photometric information available in this field. It consists of 34 sources with median redshift z = 3.7, 80 median net counts in the 0.5-7 keV band and median rest-frame absorption-corrected luminosity L_{2-10 keV} ≈ 1.5 × 10⁴⁴ erg s^-1. Spectral analysis for the full sample is presented and the intrinsic column density distribution, corrected for observational biases using spectral simulations, is compared with the expectations of X-ray background (XRB) synthesis models. We find that ≈57 per cent of the sources are highly obscured (N_H > 10²³ cm^-2). Source number counts in the 0.5-2 keV band down to flux F_{0.5-2 keV} ≈ 4 × 10^-17 erg s^-1 cm^-2 are also presented. Our results are consistent with a decline of the AGN space density at z > 3 and suggest that, at those redshifts, the AGN obscured fraction is in agreement with the expectations of XRB synthesis models.