CTQ 839: Candidate for the Smallest Projected Separation Binary Quasar

We report the discovery of the new double quasar CTQ 839. This B = 18.3, radio-quiet quasar pair is separated by 2.″1 in BRI and H filters, with magnitude differences of ΔmB = 2.5, ΔmR = ΔmI = 1.9, and ΔmH = 2.3. Spectral observations reveal both components to be z = 2.24 quasars, with relative redshifts that agree at the 100 km s-1 level but exhibit pronounced differences in the equivalent widths of related emission features, as well as an enhancement of blue continuum flux in the brighter component as compared with the fainter component longward of the Lyα emission feature. In general, similar redshift double quasars can be the result of a physical binary pair or of a single quasar multiply imaged by gravitational lensing. Empirical point-spread function subtraction of R and H band images of CTQ 839 reveal no indication of a lensing galaxy and place a detection limit of R = 22.5 and H = 17.4 for a third component in the system. For an Einstein-de Sitter cosmology and singular isothermal sphere model, the R band detection limit constrains the characteristics of any lensing galaxy to zl ≳ 1 with a corresponding luminosity of L ≳ 5 L*, while an analysis based on the redshift probability distribution for the lensing galaxy argues against the existence of a zl ≳ 1 lens at the 2 σ level. A similar analysis for a Λ-dominated cosmology, however, does not significantly constrain the existence of any lensing galaxy. The broadband flux differences, spectral dissimilarities, and failure to detect a lensing galaxy make the lensing hypothesis for CTQ 839 unlikely. The similar redshifts of the two components would then argue for a physical quasar binary. At a projected separation of 8.3 h-1 kpc (Ωm = 1), CTQ 839 would be the smallest projected separation binary quasar currently known.