Quasar clustering: evidence for an increase with redshift and implications for the nature of AGNs

The evolution of quasar clustering is investigated with a new sample of 388 quasars with 0.3 < z ≤ 2.2, B ≤ 20.5 and MB < −23, selected over an area of 24.6 deg 2 in the South Galactic Pole. Assuming a two-point correlation function of the form ξ(r) = (r/ro) , we detect clustering with r0 = 6.2 ± 1.6 h −1 comoving Mpc and ξ̄(r = 15 hMpc) = 3 r3 ∫ r 0 x ξ(x)dx = 0.52± 20 at an average redshift of < z = 1.3 >. We find a 2σ 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 favoured although it becomes less significant. With a parameterization of the evolution of the type ξ(r, z) = (r/r0) (1 + z) we find ǫ ≃ −2.5. Evolutionary parameters ǫ > 0.0 are excluded at a 0.3% probability level, to be compared with ǫ ∼ 0.8 found for galaxies. The observed clustering properties appear qualitatively consistent with a scenario of Ω = 1 CDM in which a) the difference between the quasar and the galaxy clustering can be explained as a difference in the effective bias and redshift distributions, and b) the quasars, with a lifetime of t ∼ 10yr, sparsely sample halos of mass greater than Mmin ∼ 10 12 − 10 h M⊙. We discuss also 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. Subject headings: Quasars: general — Large-scale structure of the universe Accepted for publication in the Part 1 of The Astrophysical Journal Received: 1997 August 24; Accepted: 1997 November 4 Dipartimento di Fisica, Università degli studi “Roma Tre”, Via della Vasca Navale 84, Roma, I-00146, E-mail: [email protected] Dipartimento di Astronomia, Università degli studi di Padova, Vicolo dell’Osservatorio 5, Padova, I-35122, E-mail: andreani,[email protected]