2008MNRAS.389..270R

The next generation of X-ray telescopes have the potential to detect faint quasars at very high redshift and probe the early growth of massive black holes (BHs). We present modelling of the evolution of the optical and X-ray active galactic nucleus (AGN) luminosity function at 2 < z < 6 based on a cold dark matter (CDM) merger-driven model for the triggering of nuclear activity combined with a variety of fading laws. We extrapolate the merger-driven models to z > 6 for a range of BH growth scenarios. We predict significant numbers of sources at z ∼ 6 with fluxes just an order of magnitude below the current detection limits and thus detectable with XEUS and Constellation-X, relatively independently of the fading law chosen. The predicted number of sources at even higher redshift depends sensitively on the early growth history of BHs. For passive evolution models in which BHs grow constantly at their Eddington limit, detectable BHs may be rare beyond z ∼ 10 even with Generation-X. However, in the more probable scenario that BH growth at z > 6 can be described by passive evolution with a small duty cycle, or by our merger-driven accretion model, then we predict that XEUS and Generation-X will detect significant numbers of BHs out to z ∼ 10 and perhaps beyond.