Quasar clustering: Evidence for an increase with redshift and implications for the nature of active Galactic nuclei

The evolution of quasar clustering is investigated with a new sample of 388 quasars with 0.3 < z less than or equal to 2.2, B less than or equal to 20.5, and M-B < -23, selected over an area of 24.6 deg(2) in the south Galactic pole. Assuming a two-point correlation function of the form xi(r)=(r/r(0))(-1.8), we detect clustering with r(0) = 6.2 +/- 1.6 h(-1) comoving Mpc, and <(xi)over bar> = 15 h(-1) Mpc)= 3/r(3) integral(0)(r)x(2) xi(x)dx = 0.52 +/- 20 at an average redshift of [z = 1.3]. We find a 2 sigma 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 xi(r, z)=(r/r(0))(-gamma)(1 + z)(-(3-gamma+epsilon)), we find epsilon similar or equal to -2.5. Evolutionary parameters epsilon > 0.0 are excluded at a 0.3% probability level, compared with epsilon similar to 0.8 found for galaxies. The observed clustering properties appear qualitatively consistent with a scenario of Omega = 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 similar to 10(8) yr, sparsely sample halos of mass greater than M-min similar to 10(12)-10(13) 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.