The first stars and galaxies formed when gravity overpowered the pressure of the ambient baryons. Ultimately, gravity dominated and the first stellar mass black holes were formed, very likely in gamma-ray burst explosions. Supermassive black holes can grow in cataclysmic feeding events. The highest redshift accreting black holes known are around z = 6.5. The WMAP studies of the microwave background show that the first light must have ionized the universe already as early as z = 10-20. The fact that practically all galaxy bulges in the local Universe contain supermassive black holes, with a tight relation between black hole mass and the stellar velocity dispersion, indicates a coexistence and coevolution of stars and central black holes early in the universe. Supermassive black holes must thus be an important constituent of the evolving universe. Only recently has the importance of feedback of stellar explosions and accreting black holes into the intergalactic and interstellar medium and thus their role for star and galaxy formation been realized. The study of the birth and growth of supermassive black holes at z ~ 10 requires an unprecedented combination of large spectral throughput, high angular resolving power, and large field of view in the X-ray regime, matching those of future optical and radio telescopes.
The XEUS requirement is to detect and to study X-ray emitting black holes out to z = 10 and to investigate their nature. These objects could be either continuously accreting black holes in their quasar growth phase, or, as discovered recently, mature black holes which are tidally capturing, disrupting and consuming individual stars. In either case, their X-ray luminosity should be >10425 ergs-1 in order to discriminate them from star forming emitters.
Assuming that 10% of the bolometric luminosity is emitted in X-rays and that these objects accrete at the Eddington limit, this corresponds to a black hole mass of 3 x 106M0. At z = 8, such a black hole would have an X-ray flux of 4 • 10-18 erg cm-2 s-1 (assuming standard WMAP cosmology: H0 = 70 km s-1 Mpc-1, Xo = 0.73, and a flat Universe). To study the overall properties of such objects the spectral shape, the amount of absorption, and the properties of any Fe lines need to be measured.
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