UGC 4211: A Confirmed Dual Active Galactic Nucleus in the Local Universe at 230 pc Nuclear Separation

TL;DR

This study confirms the presence of a dual active galactic nucleus in UGC 4211 at a very close nuclear separation of 230 parsecs, using multi-wavelength high-resolution observations, highlighting the potential for discovering similar systems with advanced instruments.

Contribution

First multi-wavelength high-resolution imaging and spectroscopy confirming a dual SMBH system at the smallest separation studied to date in the local universe.

Findings

Both nuclei host accreting supermassive black holes with similar masses.

The dual AGN separation is 230 pc, the closest resolved to date.

Highlights the potential of ALMA for discovering hidden SMBH pairs.

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.