Evolution of high redshift quasars and constraints on reionization

Observations of the quasar luminosity function at high redshift tell a key part of the history of massive black hole evolution. High redshift quasars are more closely connected to their seed black holes, and their rapid growth -- reaching a billion solar masses in under a Gyr -- may point to highly efficient accretion mechanisms that are relatively unchecked by feedback effects. I will summarize three recent programs that greatly enhance our understanding of the evolution of the quasar luminosity function at high redshift: 1) the BOSS survey, comprising over 20,000 quasars at 2.2 < z < 3.5 in the DR9 complete sample, 2) a survey of faint z~5 quasars in the SDSS Stripe 82 region drawn mainly from MMT spectroscopy, and 3) an HST/WFC3 Snapshot program constraining the rate of gravitational lensing in z~6 quasars. A simple picture emerges from these observations, namely that the characteristic luminosity brightens by a factor of ~10 from z=2 to z=6, while the total number density declines exponentially; this is in contrast to the pure luminosity evolution seen from z=2 to z=0 and is likely connected to changes in accretion and feedback modes near the peak of quasar activity. Finally, I will assess the contribution of quasars to reionization and revise constraints on the duration of reionization derived from quasar spectra.