The Quasar Pair Q 1634 + 267 A , B and the Binary QSO vs . Dark Lens Hypotheses

Deep HST/NICMOS H (F160W) band observations of the z = 1.96 quasar pair Q 1634+267A,B reveal no signs of a lens galaxy to a 1σ threshold of ≃ 22.5 mag. The minimum luminosity for a normal lens galaxy would be a 6L * galaxy at z ≃ 0.5, which is 1000 times greater than our detection threshold. Our observation constrains the infrared mass-to-light ratio of any putative, early-type, lens galaxy to (M/L) H ∼ > 690h 65 (1200h 65) for Ω 0 = 0.1 (1.0) and H 0 = 65h 65 km s −1 Mpc −1. We would expect to detect a galaxy somewhere in the field because of the very strong Mg ii absorption lines at z = 1.1262 in the Q 1634+267 A spectrum, but the HST H-band, I-band (F785LP) and V-band (F555W) images require that any associated galaxy be very under-luminous ∼ < 0.1L * H (1.0L * I) if it lies within ∼ < 40 h −1 65 (100h −1 65) kpc from Q 1634+267 A and B. While the large image separation (3. ′′ 86) and the lack of a lens galaxy strongly favor interpreting Q 1634+267A,B as a binary quasar system, the spectral similarity remains a puzzle. We estimate that at most 0.06% of randomly selected quasar pairs would have spectra as similar to each other as the spectra of Q 1634+267 A and B. Moreover, spectral similarities observed for the 14 quasar pairs are significantly greater than would be expected for an equivalent sample of randomly selected field quasars. Depending on how strictly we define similarity, we estimate that only 0.01–3% of randomly drawn samples of 14 quasar pairs would have as many similar pairs as the observational sample. – 2 – Subject headings: gravitational lensing – dark lens statistics – imaging – NICMOS photometry – individual: Q 1634+267 – individual: Q 1120+0195 – individual: LBQS 1429–008