Clustering analyses of 300,000 photometrically classified quasars. II. The excess on very small scales

We study quasar clustering on small scales, modeling clustering amplitudes using halo-driven dark matter descriptions. From 91 pairs on scales < 35 h(-1) kpc, we detect only a slight excess in quasar clustering over our best-fit large-scale model. Integrated across all redshifts, the implied quasar bias is b(Q) 4.21 +/- 0.98 (b(Q) 3.93 +/- 0.71) at similar to 18 h(-1) kpc (similar to 28 h(-1) kpc). Our best-fit (real space) power index is similar to-2 [i.e., xi(r) proportional to r(-2)], implying steeper halo profiles than currently found in simulations. Alternatively, quasar binaries with separation 35 h(-1) kpc may trace merging galaxies, with typical dynamical merger times t(d) similar to (610 +/- 260)m(-1/2) h(-1) Myr, for quasars of host halo mass m x 10(12) h(-1) M (circle dot). We find that UV-excess quasars at similar to 28 h(-1) kpc cluster > 5 times higher at z > 2 than at z < 2, at the 2.0 sigma level. However, as the space density of quasars declines as z increases, an excess of quasar binaries (over expectation) at z > 2 could be consistent with reduced merger rates at z > 2 for the galaxies forming UV-excess quasars. Comparing our clustering at similar to 28 h(-1) kpc to a xi(r) (r/4.8 h(-1) Mpc)(-1:53) power law, we find an upper limit on any excess of a factor of 4.3 +/- 1.3, which, noting some caveats, differs from large excesses recently measured for binary quasars, at 2.2 sigma. We speculate that binary quasar surveys that are biased to z > 2 may find inflated clustering excesses when compared to models fit at z < 2. We provide details of 111 photometrically classified quasar pairs with separations < 0.1'. Spectroscopy of these pairs could significantly constrain quasar dynamics in merging galaxies.