Abstract
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 observations, we show that the spatial
distribution, optical and NIR emission 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
dual AGN studied to date with multi-wavelength resolved spectroscopy and shows
the potential 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 at the final stages of
dynamical friction provide important constraints for future gravitational wave
observatories.
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