We present results from a spectral analysis of a sample of high-redshift ( z > 3 ) X-ray selected 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 \mathrm { keV } } \approx 1.5 \times 10 ^ { 44 } \mathrm { erg } \mathrm { 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 \approx 57 per cent of the sources are highly obscured ( N _ { H } > 10 ^ { 23 } \mathrm { cm ^ { -2 } } ) .
Source number counts in the 0.5 - 2 \mathrm { keV } band down to flux F _ { 0.5 - 2 \mathrm { keV } } \approx 4 \times 10 ^ { -17 } \mathrm { erg } \mathrm { 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 .