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 (theta < 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 less than or similar to 40 h(-1) kpc, we detect an order of magnitude excess clustering over the expectation from the large-scale (R-prop greater than or similar to 3 h(-1) Mpc) quasar correlation function, extrapolated down as a power law (gamma = 1.53) to the separations probed by our binaries. The excess grows to similar to 30 at R-prop similar to 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.