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We report the discovery of a quasar pair at z=5 separated by 21 arcsec. Bothobjects were identified as quasar candidates using simple color selectiontechniques applied to photometric catalogs from the CFHT Legacy Survey(CFHTLS). Spectra obtained with the MMT present no discernible offset inredshift between the two objects; on the other hand, there are cleardifferences in the emission line profiles and in the multiwavelength spectralenergy distributions that strongly disfavor the hypothesis that they aregravitationally lensed images of a single quasar. Both quasars are surprisinglybright given their proximity (a projected separation of ~135 kpc), with i=19.4and i=21.4. Previous measurements of the luminosity function demonstrate thatluminous quasars are extremely rare at z=5; the existence of this pair suggeststhat quasars have strong small-scale clustering at high redshift. Assuming areal-space correlation function of the form $\xi(r) \propto (r/r_0)^{-2}$, thisdiscovery implies a correlation length $r_0 > 20 h^{-1}$ Mpc, consistent with arapid strengthening of quasar clustering at high redshift as seen in previousobservations and predicted by theoretical models where feedback effects areinefficient at shutting down black hole growth at high redshift.

A Constraint on Quasar Clustering at z=5 from a Binary Quasar