A Close Quasar Pair in a Galaxy Merger at z=2.17

Distinguishing dual quasars from lensed quasars at high redshift is challenging using only optical imaging and spectroscopic techniques, because both sources have similar spectral properties. We present a comprehensive multi-wavelength analysis of a 0.5 arcsec dual quasar system, which was selected using a new “Varstrometry” technique, at z=2.166. The Hubble Space Telescope (HST) optical spectra and the Gemini near infrared (NIR) spectra confirm that both sources are quasars. The HST and Keck adaptive-optics NIR images reveal the non-detection of the lens galaxy and the faint tidal features. The flux ratios between two sources in the X-ray and radio images are significantly different than those in the optical images. These results suggest that the system is a dual quasar, instead of a lensed quasar. We find that this system is a dual quasar with the smallest projected separation (~3.8kpc) of any known dual quasars at z>1. This discovery demonstrates the effectiveness of the “Varstrometry” technique for systematically detecting kpc off-nucleus and dual quasars at cosmic noon.