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

Adam D. Myers, Robert J. Brunner, Gordon T. Richards, Robert C. Nichol, Donald P. Schneider, Neta A. Bahcall

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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 bQ = 4.21 ± 0.98 (bQ = 3.93 ± 0.71) at ∼18h-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 td ∼ (610 ± 260)m -1/2 h-1 Myr, for quasars of host halo mass m × 1012h-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.

Original languageEnglish (US)
Pages (from-to)99-106
Number of pages8
JournalAstrophysical Journal
Issue number1 I
StatePublished - Mar 20 2007

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


  • Cosmology: observations
  • Large-scale structure of universe
  • Quasars: general
  • Surveys


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