The 2dF QSO Redshift Survey - II. Structure and evolution at high redshift

In this paper we present a clustering analysis of QSOs over the redshift range z = 0.3-2.9. We use a sample of 10 558 QSOs taken from the preliminary data release catalogue of the 2dF QSO Redshift Survey (2QZ). The two-point redshift-space correlation function of QSOs, xi (Q)(s), is shown to follow a power law on scales s similar or equal to 1-35 h(-1) Mpc. Fitting a power law of the form xi (Q)(s) = (s/s(0))(-gamma) to the QSO clustering averaged over the redshift interval 0.3 < z 2.9, we find s(0) = 3.99(-0.34)(+0.28) h(-1) Mpc and gamma = 1.58(0.09)(+0.10) for an Einstein-de Sitter cosmology. The effect of a significant cosmological constant, lambda (0), is to increase the separation of QSOs, so that with Omega (0) = 0.3, lambda (0) = 0.7 the power law extends to similar or equal to 60 h(-1) Mpc and the best fit is s(0) = 5.69(-0.50)(+0.42) h(-1) Mpc and gamma = 1.56(-0.09)(+0.10). These values, measured at a mean redshift of (z) over bar = 1.49, are comparable to the clustering of local optically selected galaxies. We compare the clustering of 2QZ QSOs with generic cold dark matter (CDM) models with shape parameter Gamma (eff). Standard CDM with Gamma (eff) = 0.5 is ruled out in both Einstein-de Sitter and cosmological constant dominated cosmologies, where Gamma (eff) similar or equal to 0.2-0.4 and Gamma (eff) similar or equal to 0.1-0.2 respectively are the allowable ranges. We measure the evolution of QSO clustering as a function of redshift. For Omega (0) = 1 and lambda (0) = 0 there is no significant evolution in comoving coordinates over the redshift range of the 2QZ. QSOs thus have similar clustering properties to local galaxies at all redshifts that we sample. In the case of Omega (0) = 0.3 and lambda (0) = 0.7, QSO clustering shows a marginal increase at high redshift, s(0) being a factor of similar to1.4 higher at z similar or equal to 2.4 than at z similar or equal to 0.7. Although the clustering of QSOs is measured on large scales where linear theory should apply, the evolution of QSO clustering does not follow the linear theory predictions for growth via gravitational instability (rejected at the >99 per cent confidence level). A redshift-dependent bias is required to reconcile QSO clustering observations with theory. A simple biasing model, in which QSOs have cosmologically long lifetimes (or alternatively form in peaks above a constant threshold in the density field), is acceptable in an Omega (0) = 1 cosmology, but is only marginally acceptable if Omega (0) = 0.3 and lambda (0) = 0.7. Biasing models in which QSOs are assumed to form over a range in redshift, based on the Press-Schechter formalism, are consistent with QSO clustering evolution for a minimum halo mass of similar to 10(12) and similar to 10(13) M-circle dot in an Einstein-de Sitter and cosmological constant dominated universe, respectively. However, until an accurate, physically motivated model of QSO formation and evolution is developed, we should be cautious in interpreting the fits to these biasing models.