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 BRIH filters with magnitudedifferences of delta m_B = 2.5, delta m_R = delta m_I = 1.9, and delta m_H =2.3. Spectral observations reveal both components to be z = 2.24 quasars, withrelative redshifts that agree at the 100 km/s level, but exhibit pronounceddifferences in the equivalent widths of related emission features, as well asan enhancement of blue continuum flux in the brighter component longward of theLy alpha emission feature. In general, similar redshift double quasars can bethe result of a physical binary pair, or a single quasar multiply imaged bygravitational lensing. Empirical PSF subtraction of R and H band images of CTQ839 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-deSitter cosmology and SIS model, the R band detection limit constrains thecharacteristics of any lensing galaxy to z_lens >= 1 with a correspondingluminosity of L >~ 5 L_*, while an analysis based on the redshift probabilitydistribution for the lensing galaxy argues against the existence of a z_lens >~1 lens at the 2 sigma level. A similar analysis for a Lambda dominatedcosmology, however, does not significantly constrain the existence of anylensing galaxy. The broadband flux differences, spectral dissimilarities, andfailure to detect a lensing galaxy make the lensing hypothesis for CTQ 839unlikely. The similar redshifts of the two components would then argue for aphysical quasar binary. At a projected separation of 8.3/h kpc (Omega_matter =1), CTQ 839 would be the smallest projected separation binary quasar currentlyknown.Comment: Latex, 23 pages including 5 ps figures; accepted for publication in A