Resolving the hard X-ray background in the Chandra Deep Field South

The 1Msec observation of the Chandra Deep Field South (CDFS) has almost entirely resolved into single sources the hard X-ray background as measured by the HEAO-1 A2 experiment. The main population of sources, as determined by the first optical identifications, seems to be constituted by an admixture of type 1 and type 2 AGNs. We will consider the X-ray properties of the sources in the CDFS and test their consistency with the predictions from the AGN synthesis models of the X-ray background. An apparent excess of sources at z < 0.8 is found with respect to the model predictions. The X-ray background flux produced by these sources is also exceeding the predictions, suggesting that the bulk of the XRB is produced closer than expected.