Detecting quasars at very high redshift with next generation X-ray telescopes

TL;DR

This paper models the evolution of quasar luminosity functions to predict the detectability of high-redshift quasars with next-generation X-ray telescopes, highlighting the importance of early black hole growth scenarios.

Contribution

It introduces a merger-driven model for black hole growth and predicts the number of detectable quasars at very high redshifts with upcoming X-ray observatories.

Findings

Detectable quasars at z ~ 6 with fluxes near current detection limits.

Number of high-z sources depends on black hole growth history.

Potential to observe black holes beyond z ~ 10 with future telescopes.

Abstract

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 AGN luminosity function at 2 < z < 6 based on a 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 black holes out to z ~ 10 and perhaps beyond.