LBQS 0103-2753:: A 0".3 binary quasar

Imaging and spectroscopy with the Hubble Space Telescope show that LBQS 0103-2753 (V = 17.8, z = 0.848) is a binary quasar with a separation of 0."3, or 2.3 kpc. This is by far the smallest separation binary quasar reported to date. The two components have very different spectra, including the presence of strong broad absorption lines (BALs) in component A only. The emission-line redshifts, based on the broad high-ionization C IV lines, are z(A) = 0.834 and z(B) = 0.858; their difference is 3900 km s(-1) in velocity units. The broad C IV lines, however, are probably not a good indicator of systemic redshift, and LBQS 0103-2753A and B could have a much smaller systemic redshift difference, like the other known binary quasars. If the systemic redshift difference is small, then LBQS 0103-2753 would most likely be a galaxy merger that has led to a binary supermassive black hole. There is now one known 0."3 binary among roughly 500 QSOs that have been observed in a way that would reveal such a close binary. This suggests that QSO activity is substantially more likely for black hole binaries at spacings similar to2 kpc than at similar to 15 to 60 kpc. Between 1987 and 1998, the observed Mg II BAL disappeared.