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. 1 Echos from the past All cosmic processes like the Big Bang, the formation and evolution of galaxies, the creation and the growth of massive Black Holes and the heating of the Universe due to large-scale clustering will imprint characteristic radiation patterns on the electromagnetic spectrum of the universe. The structure of the Universe at the epoch of decoupling is frozen into the 2.7 K Cosmic Microwave Background (CMB). Star and galaxy light, produced after the Dark Ages by thermonuclear fusion, is mainly confined to the near-infrared, optical and ultraviolet spectral bands. Active black holes in galactic nuclei (AGN), which accrete matter and efficiently convert gravitational energy into radiation, typically shine in a very broad energy band, from radio to gamma wavelengths. The role of dust is also very important: it absorbs optical, UV and X-ray light, is warmed up and re-radiates the energy in the far-infrared band. The extragalactic background radiation, i.e. the total cosmic energy density spectrum, therefore gives a fossil record of all radiation processes in the universe, integrated over cosmic time. In the recent years, the extragalactic background spectrum has been determined with relatively high precision over a very broad range of the electromagnetic spectrum, apart from some inaccessible regions. Figure 1 shows a compilation of most recent determinations of the cosmic energy density spectrum from radio waves to high-energy gamma rays – the ”Echos from the Past”. Apart from the CMB peak, which clearly dominates the energy budget of the universe, three distinct components can be identified in the