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

Abstract

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, jQðsÞ; is shown to follow a power law on scales s . 1–35 h Mpc: Fitting a power law of the form jQðsÞ 1⁄4 ðs/ s0Þ to the QSO clustering averaged over the redshift interval 0:3 , z # 2:9; we find s0 1⁄4 3:99 20:34 h Mpc and g 1⁄4 1:58 20:09 for an Einstein–de Sitter cosmology. The effect of a significant cosmological constant, l0, is to increase the separation of QSOs, so that with V0 1⁄4 0:3; l0 1⁄4 0:7 the power law extends to .60 h 21 Mpc and the best fit is s0 1⁄4 5:69 20:50 h Mpc and g 1⁄4 1:56 20:09: These values, measured at a mean redshift of z 1⁄4 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 Geff. Standard CDM with Geff 1⁄4 0:5 is ruled out in both Einstein–de Sitter and cosmological constant dominated cosmologies, where Geff . 0:2–0:4 and Geff . 0:1–0:2 respectively are the allowable ranges. We measure the evolution of QSO clustering as a function of redshift. For V0 1⁄4 1 and l0 1⁄4 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 V0 1⁄4 0:3 and l0 1⁄4 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 V0 1⁄4 1 cosmology, but is only marginally acceptable if V0 1⁄4 0:3 and l0 1⁄4 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 ,10 and ,10 M( 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.