The 2dF QSO Redshift Survey - XIV. Structure and evolution from the two-point correlation function

S. M. Croom, B. J. Boyle, T. Shanks, R. J. Smith, L. Miller, P. J. Outram, N. S. Loaring, F. Hoyle, J. da Ângela

Abstract
In this paper we present a clustering analysis of quasi-stellar objects (QSOs) using over 20000 objects from the final catalogue of the 2dF QSO Redshift Survey (2QZ), measuring the redshift-space two-point correlation function, ξ(s). When averaged over the redshift range 0.3 < z < 2.2 we find that ξ(s) is flat on small scales, steepening on scales above ~25h^-1 Mpc. In a WMAP/2dF cosmology (Ω_m= 0.27, Ω_Λ= 0.73) we find a best-fitting power law with s₀= 5.48^+0.42_-0.48h^-1 Mpc and γ= 1.20 +/- 0.10 on scales s= 1 to 25h^-1 Mpc. We demonstrate that non-linear redshift-space distortions have a significant effect on the QSO ξ(s) at scales less than ~10h^-1 Mpc. A cold dark matter model assuming WMAP/2dF cosmological parameters is a good description of the QSO ξ(s) after accounting for non-linear clustering and redshift-space distortions, and allowing for a linear bias at the mean redshift of b_Q(z= 1.35) = 2.02 +/- 0.07.

We subdivide the 2QZ into 10 redshift intervals with effective redshifts from z= 0.53 to 2.48. We find a significant increase in clustering amplitude at high redshift in the WMAP/2dF cosmology. The QSO clustering amplitude increases with redshift such that the integrated correlation function, , within 20h^-1 Mpc is and . We derive the QSO bias and find it to be a strong function of redshift with b_Q(z= 0.53) = 1.13 +/- 0.18 and b_Q(z= 2.48) = 4.24 +/- 0.53. We use these bias values to derive the mean dark matter halo (DMH) mass occupied by the QSOs. At all redshifts 2QZ QSOs inhabit approximately the same mass DMHs with M_DH= (3.0 +/- 1.6) × 10¹²h^-1 M_solar, which is close to the characteristic mass in the Press-Schechter mass function, M*, at z= 0. These results imply that L*_Q QSOs at z~ 0 should be largely unbiased. If the relation between black hole (BH) mass and M_DH or host velocity dispersion does not evolve, then we find that the accretion efficiency (L/L_Edd) for L*_Q QSOs is approximately constant with redshift. Thus the fading of the QSO population from z~ 2 to ~0 appears to be due to less massive BHs being active at low redshift. We apply different methods to estimate, t_Q, the active lifetime of QSOs and constrain t_Q to be in the range 4 × 10⁶-6 × 10⁸ yr at z~ 2.

We test for any luminosity dependence of QSO clustering by measuring ξ(s) as a function of apparent magnitude (equivalent to luminosity relative to L*_Q). However, we find no significant evidence of luminosity-dependent clustering from this data set.

Monthly Notices of the Royal Astronomical Society
Volume 356, Page 415
January 2005

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