The evolution of the QSO luminosity function up to z = 4: The role of K-correction uncertainties

Uncertainties in the spectral index of the optical spectrum of QSOs induce systematic biases in the estimate of the cosmological evolution of the luminosity function. In a framework of pure luminosity evolution, we obtain a slower intrinsic evolution of the luminosity function for z < 2.2 after correction for uncertainties in the adopted K-correction. The slower evolution is consistent with the apparent density decline of high-redshift QSOs (z > 2.5) observed in the red band. Indeed, we find that a scenario of uniform luminosity evolution up to z congruent-to 4 is suggested. The apparent decline in the redshift distribution is explained in terms of the sliding down along the steep side of the luminosity function for increasing redshift.