A new assessment of the completeness of quasar surveys: Implications for the luminosity function

Abstract

We apply a simple statistical method to estimating the completeness of quasar surveys. It requires that an area has been covered by two or more, preferably different, selection techniques. We use three suitable data sets with separate selections from the following: variability and UV excess (170 quasars); objective prism and UV excess (141 quasars); and multicolor and X-ray (ROSAT, 19 quasars). We find that, for selection by UV excess, the common limit of U-B less than or equal to -0.35 +/- -0.05 leads to losses of similar to 35%, typically missing low-luminosity (M-B greater than or similar to -24.5) quasars, independently of redshift. Systematic incompleteness will therefore affect the new generation of large quasar surveys that select by U-B less than or equal to -0.35. By correcting for this incompleteness, we find from the first data set (B < 21.0 and z < 2.2) that the evolution of the quasar luminosity function (LF) is best described by joint luminosity and density evolution. When extrapolated to z = 0, the LF matches that of local Seyfert galaxies better than any previous determination. The LF shows an increase in the number of low-luminosity quasars at low redshifts and of brighter quasars at intermediate redshifts, relative to the 1990 LF of Boyle and coworkers. This result is consistent with models in which quasars fade from an initial bright phase.