X-ray Surveys of the obscured Universe
  Deep X-ray surveys have shown that the cosmic X-ray background (XRB) is largely due to accretion onto supermassive black holes, integrated over cosmic time. However, the characteristic hard spectrum of the XRB can only be explained if most AGN spectra are heavily absorbed. The absorbed AGN will suffer severe extinction and therefore, unlike classical QSOs, will not be prominent at optical wavelengths. Most of the accretion power is being absorbed by gas and dust and will have to be reradiated in the FIR/sub-mm band. AGN could therefore contribute a substantial fraction to the recently discovered cosmic FIR/sub-mm background. Here it is shown that a number of high-redshift absorbed X-ray sources selected in the ROSAT Deep Survey of the Lockman Hole have broad-band spectral energy distributions very similar to the local ULIRG NGC6240, lending additional support to the background models for the obscured universe. 
Guenther Hasinger (Astrophysikalisches Institut Potsdam)