ar X iv : a st ro - p h / 96 12 18 1 v 1 1 8 D ec 1 99 6 The QSO evolution

An Homogeneous Bright QSO Survey (HBQS) has been carried out in the framework of an ESO Key programme. 327 QSOs (with MB < −23 and 0.3 < z < 2.2) have been selected over an area of 555 deg in the magnitude range 15 < B < 18.75. For magnitudes brighter than B = 16.4 the QSO surface density turns out to be a factor 2.2 higher than what measured by the PG survey, corresponding to a surface density of 0.013 −.005 deg . If the data from the Edinburgh QSO Survey are included, an overdensity of a factor 2.7 is observed, corresponding to a surface density of 0.016±0.005 deg. In order to derive the QSO optical luminosity function we used Monte Carlo simulations that take into account of the selection criteria, photometric errors and QSO spectral slope distribution. We have combined our data with the Edinburgh QSO Survey, the Large Bright QSO Survey, Durham/AAT survey, ESO/AAT faint survey and the (ZM)B survey. The luminosity function can be represented with a Pure Luminosity Evolution, L(z) ∝ (1 + z), of a two power-law both for q0 = 0.5 and q0 = 0.1. For q0 = 0.5 the k evolution parameter is k = 3.26 ± 0.07, slower than the previous Boyle’s (1992) estimate k = 3.45. A flatter slope β = −3.72± 0.13 of the bright part of the luminosity function is also required. If a spread in the QSO spectral slope of σγ = 0.3 and 0.5 is taken into account, the k parameter drops to 3.2 and 3.0 respectively. The observed overdensity of bright QSOs is concentrated at redshifts lower than 0.6. It results that in the range 0.3 < z < 0.6 the luminosity function is flatter than observed at higher redshifts. Based on data of the ESO Key-Programme “A Homogeneous Bright QSO Survey”.