The Evolution of the Clustering of QSOs

The evolution of QSO clustering is investigated with a new sample of 388 QSOs with 0.3 < z ≤ 2.2, B ≤ 20.5 and M B < −23. Evidence is found for an increase of the clustering amplitude with increasing redshift. These measurements allow us to further distinguish between the various physical scenarios proposed to interpret the QSO phenomenon. A single population model is inconsistent with the observations. The general properties of the QSO population would arise naturally if quasars are short-lived events connected to a characteristic halo mass of 5 × 1012 M⊙.