High-redshift quasars and the epoch of reionization

Luminous quasars at high redshift provide direct probes of the evolution of supermassive black holes (SMBHs), massive galaxies, and the intergalactic medium (IGM) at early cosmic times. Discoveries of quasars at z>7 and the existence of billion-solar-mass BHs at their centers challenge theories of SMBH formation. The accelerated decline of the spatial density of luminous quasars at z>6 suggests that we are approaching the epoch when the first generation of SMBHs emerged in the early universe. Complete Gunn-Peterson absorption detected in high-redshift quasar spectra indicates a rapid increase in the IGM neutral fraction, marking the end of the epoch of reionization (EoR) at z~6. The spectra of recently discovered z>7 quasars strongly suggest that the IGM is already significantly neutral by z>7. I will review recent progress in the surveys of the most distant quasars, and in using them as probe to early SMBH growth, BH/galaxy co-evolution and the history of reionization. I will also discuss the prospect of future EoR quasar surveys in discovering the earliest luminous quasars in the universe.