The Chandra deep field South/GOODS survey -: Optically faint X-ray sources

We provide important new constraints on the nature and redshift distribution of optically faint ( R >= 25) X- ray sources in the Chandra Deep Field South Survey. We use a large multi- wavelength data set, including the GOODS/ ACS survey, the recently released Hubble Ultra Deep Field ( UDF) data, and the new public VLT/ ISAAC imaging. We show that we can derive accurate photometric redshifts for the spectroscopically unidentified sources thus maximizing the redshift completeness for the whole X- ray sample. Our new redshift distribution for the X- ray source population is in better agreement with that predicted by X- ray background synthesis models; however, we still find an overdensity of low redshift ( z < 1) sources. The optically faint sources are mainly X- ray absorbed AGN, as determined from direct X- ray spectral analysis and other diagnostics. Many of these optically faint sources have high (> 10) X- ray- to- optical flux ratios. We also find that similar to 71% of them are well fitted with the SED of an early- type galaxy with < z(phot)> similar to 1.9 and the remaining 29% with irregular or starburst galaxies mainly at z(phot) > 3. We estimate that 15% of the optically faint sources are X- ray absorbed QSOs. The overall population of X- ray absorbed QSOs contributes a similar to 11% fraction of the [ 2 - 10] keV X- ray Background ( XRB) whereas current XRB synthesis models predict a similar to 38% contribution.