Binary quasars in the sloan digital sky survey: Evidence for excess clustering on small scales

We present a sample of 221 new quasar pairs with proper transverse separations R _prop < 1 h ^-1 Mpc over the redshift range 0.5 < z < 3.0, discovered from an extensive follow-up campaign to find companions around the Sloan Digital Sky Survey and 2dF QSO Redshift Survey quasars. This sample includes 26 new binary quasars with separations R _prop < 50 h ^-1 kpc (θ < 10″), more than doubling the number of such systems known. We define a statistical sample of binaries selected with homogeneous criteria and compute its selection function, taking into account sources of incompleteness. The first measurement of the quasar correlation function on scales 10 h ^-1 kpc < R _prop < 400 h ^-1 kpc is presented. For R _prop ≲ 40 h ^-1 kpc, we detect an order of magnitude excess clustering over the expectation from the large-scale (R _prop ≳ 3 h ^-1 Mpc) quasar correlation function, extrapolated down as a power law (γ = 1.53) to the separations probed by our binaries. The excess grows to ∼30 at R _prop 10 h ^-1 kpc and provides compelling evidence that the quasar autocorrelation function gets progressively steeper on submegaparsec scales. This small-scale excess can likely be attributed to dissipative interaction events that trigger quasar activity in rich environments. Recent small-scale measurements of galaxy clustering and quasar-galaxy clustering are reviewed and discussed in relation to our measurement of small-scale quasar clustering.