Search

69 pages, 21 figures. Accepted for publication in New Astronomy Reviews © 2020 Elsevier B.V., The quest for binary and dual supermassive black holes (SMBHs) at the dawn of the multi-messenger era is compelling. Detecting dual active galactic nuclei (AGN) -- active SMBHs at projected separations larger than several parsecs -- and binary AGN -- probing the scale where SMBHs are bound in a Keplerian binary -- is an observational challenge. The study of AGN pairs (either dual or binary) also represents an overarching theoretical problem in cosmology and astrophysics. The AGN triggering calls for detailed knowledge of the hydrodynamical conditions of gas in the imminent surroundings of the SMBHs and, at the same time, their duality calls for detailed knowledge on how galaxies assemble through major and minor mergers and grow fed by matter along the filaments of the cosmic web. This review describes the techniques used across the electromagnetic spectrum to detect dual and binary AGN candidates and proposes new avenues for their search. The current observational status is compared with the state-of-the-art numerical simulations and models for formation of dual and binary AGN. Binary SMBHs are among the loudest sources of gravitational waves (GWs) in the Universe. The search for a background of GWs at nHz frequencies from inspiralling SMBHs at low redshifts, and the direct detection of signals from their coalescence by the Laser Interferometer Space Antenna in the next decade, make this a theme of major interest for multi-messenger astrophysics. This review discusses the future facilities and observational strategies that are likely to significantly advance this fascinating field.© 2020 Elsevier B.V., All authors acknowledge the hospitality of the Lorentz Center for international workshops (Leiden, The Netherlands), where the idea of this work was born, and acknowledge the support of the International Space Science Institute (ISSI Bern, Switzerland), where the collaboration was originated. We thank the reviewers for having provided valuable and constructive comments that improved the clarity of the manuscript, and J.E. Barnes, L. Blecha, M. Eracleous, B.D. Farris, H. Fu, X. Liu, R. Pfeifle, L.C. Popovic, C. Ricci, S. Rodriguez, S. Tang, G. B. Taylor for their kind permission to reuse figures from their publications. ADR, CV and SB acknowledge financial support from ASI under grant ASI-INAF I/037/12/0 , and from the agreement ASI-INAF n. 2017-14-H.O . TB acknowledges support by the National Aeronautics and Space Administration under Grant No. NNX15AK84G and 80NSSC19K0319 issued through the Astrophysics Theory Program and by the Research Corporation for Science Advancement through a Cottrell Scholar Award. SF and KÉG thank the Hungarian National Research, Development, and Innovation Office ( OTKA NN110333 ) for support. KÉG was supported by the János Bolyai Research Scholarship of the Hungarian Academy of Sciences and by the ÚNKP-19-4 New National Excellence Program of the Ministry for Innovation and Technology. BH acknowledges financial support by the DFG grant GE625/17-1 . EL is supported by a European Union COFUND/Durham Junior Research Fellowship (under EU grant agreement no. 609412). DL acknowledges support from the European Research Council (ERC) under grant 647208 (PI Jonker). AS is supported by the ERC CoG grant 818691 (B Massive). MGi is supported by the “Programa de Atracción de Talento” of the Comunidad de Madrid grant 2018-T1/TIC-11733 for the project: “Unveiling Black Hole Winds from Space”, and by the Spanish State Research Agency (AEI) grant MDM-2017-0737 Unidad de Excelencia “María de Maeztu” - Centro de Astrobiología (INTA-CSIC). PRC and LM acknowledge support from the Swiss National Science Foundation under the Grant 200020_178949 . MC acknowledges support from the National Science Foundation (NSF) NANOGrav Physics Frontier Center, award number 1430284 . NHR acknowledges support from the BMBF Verbundforschung under FKZ: 05A17PC1 and FKZ: 05A17PC2. ZH acknowledges support from NASA grants NNX17AL82G and 80NSSC19K0149 and NSF grant 1715661 . KI acknowledges support by the Spanish MINECO under grant AYA2016-76012-C3-1-P and MDM-2014-0369 of ICCUB (Unidad de Excelencia ’María de Maeztu’). MPT acknowledges financial support from the Spanish MCIU through the “Center of Excellence Severo Ochoa” award for the Instituto de Astrofísica de Andalucía ( SEV-2017-0709 ) and through the MINECO grants AYA2012-38491-C02-02 and AYA2015-63939-C2-1-P .