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Published January 1, 2023 | Published
Journal Article Open

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


We present multiwavelength 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/Space Telescope Imaging Spectrograph, Very Large Telescope/MUSE+AO, Keck/OSIRIS+AO spectroscopy, and the Atacama Large Millimeter/submillimeter Array (ALMA) observations, we show that the spatial distribution, optical and near-infrared 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, suggest that both SMBHs have similar masses, log(M_(BH)/M_⊙) ∼ 8.1 (south) and log(M_(BH)/M_⊙) ∼ 8.3 (north), respectively. The projected separation of 230 pc (∼6× the black hole sphere of influence) represents the closest-separation dual active galactic nuclei (AGN) studied to date with multiwavelength 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 subkiloparsec regime at the final stages of dynamical friction provide important constraints for future gravitational wave observatories.

Additional Information

© 2023. The Author(s). Published by the American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. We thank the anonymous referee for very useful comments and suggestions to improve the manuscript. The authors would like to express their appreciation for useful software discussions with Roland Bacon and Yao Yao as well as the observation planning with ESO astronomer Michael Hilker who supported multiple attempts to observe this object. We acknowledge support from: NASA through ADAP award NNH16CT03C (M.K.), ANID through Millennium Science Initiative Program-NCN19_058 (E.T.), and ICN12_009 (F.E.B.), CATA-BASAL - ACE210002 (E.T., F.E.B.), and FB210003 (E.T., F.E.B., C.R.), FONDECYT Regular-1190818 (E.T., F.E.B.), and 1200495 (F.E.B., E.T.), and Fondecyt Iniciacion 11190831 (C.R.), NSF grants PHY-2020265 (T.L., C.M.F.M.) and AST-2106552 (C.M.F.M.), NSF award AST-1909933 and Cottrell Scholar Award 27553 (L.B.), ADAP award 80NSSC19K1096 (F.M.-S.), the European Union's Horizon 2020 research and innovation program (950533), and Israel Science Foundation grant 1849/19 (B.T.), Project No. 2022-1-830-06 from the Korea Astronomy and Space Science Institute (K.O.) and the National Research Foundation of Korea (NRF-2020R1C1C1005462), JSPS KAKENHI grant JP20K14529 and the Special Postdoctoral Researchers Program at RIKEN (T.K.), and the hospitality of NRAO/NAASC during his sabbatical leave (E.T.). The Flatiron Institute is supported by the Simons Foundation. Facilities: ALMA - Atacama Large Millimeter Array, CXO - , HST (STIS - , ACS - , WFC3) - , Keck:I (OSIRIS) - , Keck:II (NIRC2) - , NuSTAR - , Swift (BAT and XRT) - , VLA - , VLT:Yepun. - Software: APLpy (Robitaille & Bressert 2012), astropy (Collaboration et al. 2013), Matplotlib (Hunter 2007), jupyter notebook, Numpy (van der Walt et al. 2011), MPDAF (Bacon et al. 2016), pPXF (Cappellari 2017), PySpecKit (Ginsburg & Mirocha 2011), vorbin (Cappellari & Copin 2003).

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August 22, 2023
August 22, 2023