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The faint end of the luminosity function for QSOs is estimated from a spectroscopic sample complete to B = 21.1 and unbiased to B = 22.6, the faintest survey so far. A surface density of 100 QSOs per square degree is obtained from 30 bona fide QSOs found in 0.29 square degrees at the north Galactic pole (SA 57). A turnover in the slope of the luminosity function is evident when bright and faint samples are combined. The appearance of this change in slope in the luminosity function provides an important constraint on physical models for QSO evolution. The dependence on redshift of this feature in the luminosity-density diagram is continuous, and we see no evidence for a "cutoff" in redshift at any luminosity. We propose a new model for the evolution that adequately describes the data for z > 1. We determine the emissivity due to QSOs as a function of redshift, and show that, with conventional assumptions, the observed QSOs cannot explain the level of ionization of the intergalactic medium implied by the Gunn-Peterson test.
Koo et al. (Mon,) studied this question.