Discovery of a Radio-loud/Radio-quiet Binary Quasar

Abstract

We report the discovery of a small-separation quasar pair (z=0.586, O=18.4, 19.2, and a separation of 23) associated with the radio source FIRST J164311.3+315618 (S1400=120 mJy). The spectrum of the brighter quasar (A) has a much stronger narrow emission line spectrum than the other (B), and it also has stronger Balmer lines relative to the continuum. The continuum ratio of the spectra is flat in the blue (λobs<6000 A) at about 2.1 but falls to 1.5 at longer wavelengths. A K' image shows two unresolved sources with a flux ratio of 1.3. The different colors appear to result from the contribution of the host galaxy of B, which is evident from Ca II and high-order Balmer absorption lines that are indicative of a substantial young stellar population. New 3.6 cm VLA observations show that the compact radio source is coincident with quasar A (B is only marginally detected). We rule out the lensing hypothesis because the optical flux ratio is A/B≈1.5-2, while the radio flux ratio is A/B40, and conclude that this system is a binary. Moreover, the radio-loud quasar is a compact, steep spectrum source. FIRST J164311.3+315618A, B is the lowest redshift and smallest separation binary quasar yet identified.