Quasar Clustering: Evidence for an Increase with Redshift and Implications for the Nature of Active Galactic Nuclei

Abstract

The evolution of quasar clustering is investigated with a new sample of 388 quasars with 0.3 < z ≤ 2.2, B ≤ 20.5, and M_B < -23, selected over an area of 24.6 deg² in the south Galactic pole. Assuming a two-point correlation function of the form ξ(r) = (r/r_o)^-1.8, we detect clustering with r₀ = 6.2 ± 1.6 h^-1 comoving Mpc, and (r=15 h^-1 Mpc)=3/r³ ∫₀^r x²ξ(x)dx=0.52±20 at an average redshift of z = 1.3. We find a 2 σ significant increase of the quasar clustering between z = 0.95 and z = 1.8, independent of the quasar absolute magnitude and inconsistent with recent evidence on the evolution of galaxy clustering. If other quasar samples are added (resulting in a total data set of 737 quasars), the increase of the quasar clustering is still favored, although it becomes less significant. With a parameterization of the evolution of the type ξ(r, z) = (r/r₀)^-γ(1 + z)^-(3-γ+), we find -2.5. Evolutionary parameters > 0.0 are excluded at a 0.3% probability level, compared with ~ 0.8 found for galaxies. The observed clustering properties appear qualitatively consistent with a scenario of Ω = 1 cold dark matter in which (1) the difference between the quasar and the galaxy clustering can be explained as a difference in the effective bias and redshift distributions and (2) the quasars, with a lifetime of t ~ 10⁸ yr, sparsely sample halos of mass greater than M_min ~ 10¹²-10¹³ h^-1 M_☉. We also discuss the possibility that the observed change in the quasar clustering is due to an increase in the fraction of early-type galaxies as quasar hosts at high z.