A snapshot survey for gravitational lenses among z ≥ 4.0 quasars. II. Constraints on the 4.0 < z < 5.4 quasar population

We report on i-band snapshot observations of 157 Sloan Digital Sky Survey quasars at 4.0 -3.8 (3 σ), assuming a break in the quasar luminosity function at M ~ -24.5. This constraint is considerably stronger than the limit of β > -4.63 obtained from the absence of lensing in four z > 5.7 quasars. Such constraints are important to our understanding of the true space density of high-redshift quasars and the ionization state of the early universe.

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