SIMBAD references

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 L2-10keV ~ 1.5x1044ergs-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 (NH > 1023cm-2). Source number counts in the 0.5-2keV band down to flux F0.5-2keV ~ 4x10-17ergs-1cm-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.