The X-Ray Background
Gyula Szokoly
We present the results of our VLT spectroscopic followups of the X-ray
sources detected in the 942 ks exposure of the Chandra Deep Field South.
Among of the 349 Chandra sources, spectroscopic redshifts were obtained for
168 X-ray sources, of which 137 have both reliable optical identification and
redshift estimate. The type-1 and type-2 populations are clearly separated in
X-ray/optical diagnostics, but 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, hardness ratio and X-ray luminosity, consistent with the unified
AGN model. There is a strong decrease at z<2 in the fraction of luminous
type-2 QSOs. A large population of EROs (R-K>5) as X-ray counterparts covers
the whole range of X-ray hardness ratios, comprise objects of various classes
(absorbed AGN, elliptical and starburst galaxies.