The quasar pair Q1634+267A,B and the binary QSO versus dark lens hypotheses

Deep HST/NICMOS H-band (F160W) observations of the z = 1.96 quasar pair Q1634+267A and B reveal no signs of a lens galaxy to a 1 σ threshold of ≃22.5 mag. The minimum luminosity for a normal lens galaxy would be a 6L* galaxy at z ≃ 0.5, which is 650 times greater than our detection threshold. Our observation constrains the infrared mass-to-light ratio (M/L) of any putative, early-type, lens galaxy to (M/L)_H ≳ 690 h₆₅ (1200 h₆₅) for Ω₀ = 0.1 (1.0) and H₀ = 65 h₆₅ 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 Q1634+267A spectrum, but the HST H-band, I-band (F785LP), and V-band (F555W) images require that any associated galaxy be very underluminous ≲0.1L*_H (1.0L*_I) if it lies within ≲40 h^-1₆₅ (100 h^-1₆₅) kpc from Q1634+267A and B. While the large image separation (3″.85) and the lack of a lens galaxy strongly favor interpreting Q1634+267A and 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 Q1634+267A and B. Moreover, spectral similarities observed for the 14 known 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.