Cosmic evolution of quasar clustering: implications for the host haloes

We present detailed clustering measurements from the 2dF Quasi-Stellar Object Redshift Survey (2QZ) in the redshift range 0.8 < z < 2.1. Using a flux-limited sample of similar to14000 objects with effective redshift z(eff) = 1.47, we estimate the quasar projected correlation function for separations 1 < r/ h(-1) Mpc < 20. We find that the two-point correlation function in real space is well approximated by a power law with slope y = 1.5 +/- 0.2 and comoving correlation length r(0) = 4.8-(+0-9)(1.5) h-1 Mpc. Splitting the sample into three subsets based on redshift, we find evidence for an increase of the clustering amplitude with look-back time. For a fixed y, evolution of r(0) is detected at the 3.6sigma confidence level. The ratio between the quasar correlation function and the mass autocorrelation function (derived adopting the concordance cosmological model) is found to be scale-independent. For a linear mass-clustering amplitude sigma(8) = 0.8, the 'bias parameter' decreases from b similar or equal to 3.9 at z(eff) = 1.89 to b similar or equal to 1.8 at z(eff) = 1.06. From the observed abundance and clustering, we infer how quasars populate dark matter haloes of different masses. We find that 2QZ quasars sit in haloes with M > 10(12) Me and that the characteristic mass of their host haloes is of the order of 1013 Me. The observed clustering is consistent with assuming that the locally observed correlation between black hole mass and host galaxy circular velocity is still valid at z > 1. From the fraction of haloes which contain active quasars, we infer that the characteristic quasar lifetime is t(Q) similar to a few x 10(7) yr at z similar to 1 and approaches 108 yr at higher redshifts.