SIMBAD references

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 ∼71% of them are well fitted with the SED of an early-type galaxy with <z_phot≳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 ∼11% fraction of the [2-10]keV X-ray Background (XRB) whereas current XRB synthesis models predict a ∼38 % contribution.