A Constraint on Quasar Clustering at z=5 from a Binary Quasar
  We report the discovery of a quasar pair at z=5 separated by 21 arcsec. Both
objects were identified as quasar candidates using simple color selection
techniques applied to photometric catalogs from the CFHT Legacy Survey
(CFHTLS). Spectra obtained with the MMT present no discernible offset in
redshift between the two objects; on the other hand, there are clear
differences in the emission line profiles and in the multiwavelength spectral
energy distributions that strongly disfavor the hypothesis that they are
gravitationally lensed images of a single quasar. Both quasars are surprisingly
bright given their proximity (a projected separation of ~135 kpc), with i=19.4
and i=21.4. Previous measurements of the luminosity function demonstrate that
luminous quasars are extremely rare at z=5; the existence of this pair suggests
that quasars have strong small-scale clustering at high redshift. Assuming a
real-space correlation function of the form $\xi(r) \propto (r/r_0)^{-2}$, this
discovery implies a correlation length $r_0 > 20 h^{-1}$ Mpc, consistent with a
rapid strengthening of quasar clustering at high redshift as seen in previous
observations and predicted by theoretical models where feedback effects are
inefficient at shutting down black hole growth at high redshift.
