200-Mpc-sized structure in the 2dF QSO Redshift survey

Abstract

The completed 2dF QSO Redshift (2QZ) survey has been used to search for extreme large-scale cosmological structure (∼200 h−1 Mpc) over the redshift range 0 < z < 2.5. We demonstrate that statistically significant overdensities and underdensities do exist and hence represent the detection of cosmological fluctuations on comoving scales that correspond to those presently detected in the cosmic microwave background. However, the fractional overdensities on scales >100 h−1 Mpc are in the linear or only weakly non-linear regime and do not represent collapsed non-linear structures. We compare the measurements with the expectation of the Λ-cold dark matter (ΛCDM) model by measuring the variance of counts in cells and we find that, provided the distribution of quasi-stellar objects (QSOs) on large scales exhibits a mild bias with respect to the distribution of dark matter, the observed fluctuations are found to be in good agreement with the model. There is no evidence on such scales for any extreme structures that might require, for example, departures from the assumption of Gaussian initial perturbations. Thus, the power spectrum derived from the 2QZ survey appears to provide a complete description of the distribution of QSOs. The amount of bias and its redshift dependence that is required is consistent with that found from studying the clustering of 2QZ QSOs on ∼10 h−1 Mpc scales, and may be adequately described by an approximately redshift-invariant power spectrum with normalization σ8≃ 1.0 corresponding to a bias at z= 0 of b≃ 1.1 rising to b≃ 2 at the survey's mean redshift z≃ 1.5.