2007ApJ...658...99M

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 ∼18 h^–1 kpc (∼28 h^–1 kpc). Our best-fit (real space) power index is ~-2 [i.e., ξ(r)∝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~(610±260)m^–1/2 h^–1 Myr, for quasars of host halo mass mx10¹² h^–1 M_☉. We find that UV-excess quasars at ∼28 h^–1 kpc cluster >5 times higher at z>2 than at z<2, at the 2.0 σ 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 ∼28 h^–1 kpc to a ξ(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 σ. 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.