The Chandra Deep Field–South: Optical Spectroscopy. I.

We present the results of our spectroscopic follow-up program of the X-ray sources detected in the 942 ks exposure of the Chandra Deep Field - South (CDFS). A total of 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 colors 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 active galactic nuclei ( AGNs) but also type 1 AGN and X-ray normal galaxies: the one 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 sheetlike 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 color, 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) ergs s(-1)] at z > 2 (13 sources with unambiguous spectroscopic identification); most X-ray type 1 QSOs are bright, Rless than or similar to24, whereas most X-ray type 2 QSOs have Rgreater than or similar to24, 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 similar to 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 color of most X- ray type 2 AGNs 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 AGNs. 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 greater than or equal to 24 sample. They cover the whole range of X- ray hardness ratios, comprise objects of various classes ( in particular a high fraction of zgreater than or similar to1 X- ray absorbed AGNs, but also elliptical and starburst galaxies) and more than half of them should be fairly bright X- ray sources [ LX( 0: 5 10 keV) > 1042 ergs s(-1)]. Photometric redshifts will be necessary to derive the properties and evolution of the X- ray selected EROs.