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 Q16 34+267A and B reveal no signs of a lens galaxy to a 1 sigma threshold of si milar or equal to 22.5 mag. The minimum luminosity for a normal lens galaxy would be a 6L* galaxy at z similar or equal to 0.5, which is 650 times gre ater than our detection threshold. Our observation constrains the infrared mass-to-light ratio (MIL) of any putative, early-type, lens galaxy to (MIL) , greater than or similar to 690 h(65) (1200 h(65)) for Omega(0) = 0.1 (1.0 ) and H-0 = 65 h(65) km s(-1) Mpc(-1). We would expect to detect a galaxy s omewhere 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 under luminous less than or similar to 0.1L*(H) (1.0L*(I)) if it lies within less than or similar to 40 h(65)(-1) (100 h(65)(-1)) kpc from Q1634+267A and B. While the large image separation (3 ".85) and the lack of a lens galaxy st rongly 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 observe d for the 14 known quasar pairs are significantly greater than would be exp ected for an equivalent sample of randomly selected field quasars. Dependin g on how strictly we define similarity, we estimate that only 0.01%-3% of r andomly drawn samples of 14 quasar pairs would have as many similar pairs a s the observational sample.