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 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 bQ = 4.21± 0.98 (bQ = 3.93± 0.71) at ∼18 h −1 kpc (∼28 h kpc). Our best-fit (real-space) power index is ∼-2 (i.e., ξ(r) ∝ r), implying steeper halo profiles than currently found in simulations. Alternatively, quasar binaries with separation < 35 h kpc may trace merging galaxies, with typical dynamical merger times td ∼ (610±260)m hMyr, for quasars of host halo massm×10 hM⊙. We find UVX quasars at ∼28 h kpc cluster > 5 times higher at z > 2, than at z < 2, at the 2.0σ 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 UVX quasars. Comparing our clustering at ∼28 h kpc to a ξ(r) = (r/4.8 h Mpc) 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 2.2σ. 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. Subject headings: cosmology: observations — large-scale structure of universe — quasars: general — surveys