We present the results of our spectroscopic follow-up program of the X-ray sources detected in the 942 ks exposure of the C handra D eep F ield S outh ( CDFS ) .
288 possible counterparts were observed at the VLT with the FORS1/FORS2 spectrographs for 251 of the 349 Chandra sources ( including three additional faint X-ray sources ) .
Spectra and R-band images are shown for all the observed sources and R - K colours are given for most of them .
Spectroscopic redshifts were obtained for 168 X-ray sources , of which 137 have both reliable optical identification and redshift estimate ( including 16 external identifications ) .
The R < 24 
observed sample comprises 161 X-ray objects ( 181 optical counterparts ) and 126 of them have unambiguous spectroscopic identification .
There are two spikes in the redshift distribution , predominantly populated by type-2 AGN but also type-1 AGN and X-ray normal galaxies : that at z = 0.734 is fairly narrow ( in redshift space ) and comprises two clusters/groups of galaxies centered on extended X-ray sources , the second one at z = 0.674 is broader and should trace a sheet-like structure .
The type-1 and type-2 populations are clearly separated in X-ray/optical diagnostics involving parameters sensitive to absorption/reddening : X-ray hardness ratio ( HR ) , optical/near-IR colour , soft X-ray flux and optical brightness .
Nevertheless , these two populations cover similar ranges of hard X-ray luminosity and absolute K magnitude , thus trace similar levels of gravitational accretion .
Consequently , we introduce a new classification based solely on X-ray properties , HR and X-ray luminosity , consistent with the unified AGN model .
This X-ray classification uncovers a large fraction of optically obscured , X-ray luminous AGNs missed by the classical optical classification .
We find a similar number of X-ray type-1 and type-2 QSOs ( L _ { X } ( 0.5-10 keV ) > 10 ^ { 44 } erg s ^ { -1 } ) at z > 2 ( 13 sources with unambiguous spectroscopic identification ) ; most X-ray type-1 QSOs are bright , R \lesssim 24 , whereas most X-ray type-2 QSOs have R \gtrsim 24 which may explain the difference with the CDFN results as few spectroscopic redshifts were obtained for R > 24 CDFN X-ray counterparts .
There are X-ray type-1 QSOs down to z \sim 0.5 , but a strong decrease at z < 2 in the fraction of luminous X-ray type-2 QSOs may indicate a cosmic evolution of the X-ray luminosity function of the type-2 population .
An X-ray spectral analysis is required to confirm this possible evolution .
The red colour of most X-ray type-2 AGN could be due to dust associated with the X-ray absorbing material and/or a substantial contribution of the host galaxy light .
The latter can also be important for some redder X-ray type-1 AGN .
There is a large population of EROs ( R - K > 5 ) as X-ray counterparts and their fraction strongly increases with decreasing optical flux , up to 25 % for the R \geq 24 sample .
They cover the whole range of X-ray hardness ratios , comprise objects of various classes ( in particular a high fraction of z \gtrsim 1 X-ray absorbed AGNs , but also elliptical and starburst galaxies ) and more than half of them should be fairly bright X-ray sources ( L _ { X } ( 0.5-10 keV ) > 10 ^ { 42 } 
erg s ^ { -1 } ) .
Photometric redshifts will be necessary to derive the properties and evolution of the X-ray selected EROs .