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 10558 QSOs taken from the preliminary data release catalogue of the 2dF QSO Redshift Survey (2QZ). The two-point redshift-space correlation function of QSOs, ξ_Q(s), is shown to follow a power law on scales s~=1-35h^-1Mpc. Fitting a power law of the form ξ_Q(s)=(s/s₀)^-γ to the QSO clustering averaged over the redshift interval 0.3<z<=2.9, we find s₀=3.99-0.34+0.28h^-1Mpc and γ=1.58-0.09+0.10 for an Einstein-de Sitter cosmology. The effect of a significant cosmological constant, λ₀, is to increase the separation of QSOs, so that with Ω₀=0.3, λ₀=0.7 the power law extends to ~=60h^-1Mpc and the best fit is s₀=5.69-0.50+0.42h^-1Mpc and γ=1.56-0.09+0.10. These values, measured at a mean redshift of z=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 Γ_eff. Standard CDM with Γ_eff=0.5 is ruled out in both Einstein-de Sitter and cosmological constant dominated cosmologies, where Γ_eff~=0.2-0.4 and Γ_eff~=0.1-0.2 respectively are the allowable ranges. We measure the evolution of QSO clustering as a function of redshift. For Ω₀=1 and λ₀=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 Ω₀=0.3 and λ₀=0.7, QSO clustering shows a marginal increase at high redshift, s₀ being a factor of ~1.4 higher at z~=2.4 than at z~=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 Ω₀=1 cosmology, but is only marginally acceptable if Ω₀=0.3 and λ₀=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 ~10¹² and ~10¹³M_solar 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.