The 2dF QSO Redshift Survey - I. The optical luminosity function of quasi-stellar objects

We present a determination of the optical luminosity function of quasi-stellar objects (QSOs) and its cosmological evolution with redshift for a sample of over 6000 QSOs identified primarily from the first observations of the 2dF QSO Redshift Survey (2QZ). For QSOs with -26<M_B<-23 and 0.35<z<2.3, we find that pure luminosity evolution (PLE) models provide an acceptable fit to the observed redshift dependence of the luminosity function (LF). The LF is best fitted by a two-power-law function of the form φ(L_B) ∝ [(L_BL^*_B)^α + (L_BL^*_B)^β]^-1. Exponential luminosity evolution models, both as a function of look-back time, L^*_B(z) = L^*_B(0)e^k1tau, and as a general second-order polynomial, L^*_B(z) ∝ 10^{k1z+k2z^2}, were found to provide acceptable fits to the data set comprising the 2QZ and the Large Bright Quasar Survey. Exponential evolution with look-back time is preferred for q₀=0.05, while the polynomial evolution model is preferred for q₀=0.5. The shape and evolution of the LF at low redshifts (z<0.5) and/or high luminosities, not currently well sampled by the 2QZ survey, may show departures from pure luminosity evolution, but the results presented here show that, over a significant range of redshift, PLE is a good description of QSO evolution.