ROSAT/ASCA deep surveys

ROSAT deep and shallow surveys have provided an almost complete inventory of the constituents of the soft X-ray background which led to a population synthesis model for the whole X-ray background with interesting cosmological consequences. According to this model the X-ray background is the "echo" of mass accretion onto supermassive black holes with absorbing molecular tori, integrated over cosmic time. Deep surveys with ASCA are in agreement with this model, a nem determination of the soft X-ray luminosity function of active galactic nuclei (AGN) is consistent with pure density evolution, and the comoving volume density of AGN at redshift 2-3 approaches that of local normal galaxies. This indicates that many larger galaxies contain black holes, which were active in the past. The cosmic history of black hole density and star forming rate indicates, that the bulk of black holes mas produced before most of the stars in the universe. However, only more sensitive and higher angular resolution X-ray surveys in the harder energy bands, where the maximum of the energy density of the X-ray background resides, will provide the acid test of this picture.