A Confirmed Dual AGN at 230 pc Separation

We present multi-wavelength high-spatial resolution (~0.1″, 70 pc) observations of UGC 4211 at z=0.03474, a late-stage major galaxy merger at the closest nuclear separation yet found in near-IR imaging (0.32″, ~230 pc projected separation). Using Hubble Space Telescope/STIS, VLT/MUSE+AO, Keck/OSIRIS+AO spectroscopy, and ALMA continuum observations, we show that the spatial distribution, emission and stellar absorption lines, and millimeter continuum emission are all consistent with both nuclei being powered by accreting supermassive black holes (SMBHs). Our data, combined with common black hole mass prescriptions, suggests that both SMBHs have similar masses, log MBH~8.1 (south) and log MBH~8.3 (north), respectively. The projected separation of 230 pc (~6X the black hole sphere of influence) represents the closest-separation confirmed dual AGN studied to date with multi-wavelength resolved spectroscopy and shows the effectiveness of nuclear (<50 pc) continuum observations with ALMA to discover hidden growing SMBH pairs. While the exact occurrence rate of close-separation dual AGN is not yet known, it may be surprisingly high, given that UGC 4211 was found within a small, volume-limited sample of nearby hard X-ray detected AGN. Observations of dual SMBH binaries in the sub-kpc regime in the final stages of dynamical friction provide important constraints for future gravitational wave observatories.