Varstrometry for Off-nucleus and Dual Subkiloparsec AGN (VODKA): Hubble Space Telescope Discovers Double Quasars

Abstract Dual supermassive black holes (SMBHs) at ?kiloparsec scales are the progenitor population of SMBH mergers and play an important role in understanding pairing dynamical evolution massive galaxy mergers. Because stringent resolution requirement apparent rareness these small-separation pairs, there scarce observational constraints on this population, with few confirmed dual SMBHs <10 kpc separations z > 1. Here we present results from a pilot search for kiloparsec-scale quasars selected Gaia Data release 2 (DR2) astrometry followed up Hubble Space Telescope (HST) Wide Field Camera 3 dual-band (F475W F814W) snapshot imaging. Our targets primarily varstrometry technique, i.e., light centroid jitter caused by asynchronous variability both members unresolved quasar pair, supplemented subarcsecond pairs already resolved DR2. We find overall high fraction HST-resolved among varstrometry-selected (unresolved DR2), ?30%–50%, increasing toward redshift (?60%–80% 1.5). discuss nature 45 based HST supplementary data. A substantial (?40%) likely physical or gravitationally lensed quasars. also discover triple candidate quadruply quasar, which is smallest-separation quadruple lenses. These provide guidelines to improve selection follow-up confirmation ~kiloparsec-scale redshift.