New Astronomy Reviews 86 (2019) 101525 Contents lists available at ScienceDirect New Astronomy Reviews journal homepage: www.elsevier.com/locate/newastrev The quest for dual and binary supermassive black holes: A multi-messenger T view ⁎ Alessandra De Rosa ,a, Cristian Vignalib,c, Tamara Bogdanovićd, Pedro R. Capeloe, Maria Charisif, Massimo Dottig,h, Bernd Husemanni, Elisabeta Lussoj,k,l, Lucio Mayere, Zsolt Paragim, Jessie Runnoen,o, Alberto Sesanag,p, Lisa Steinbornq, Stefano Bianchir, Monica Colpig, Luciano del Valles, Sándor Freyt, Krisztina É. Gabányit,u,v, Margherita Giustiniw, Matteo Guainazzix, Zoltan Haimany, Noelia Herrera Ruizz, Rubén Herrero-IllanaA,B, Kazushi IwasawaC, S. KomossaD, Davide LenaE,F, Nora LoiseauG, Miguel Perez-TorresH, Enrico PiconcelliI, Marta Volonteris a INAF/IAPS - Istituto di Astrofsica e Planetologia Spaziali, Via del Fosso del Cavaliere I-00133, Roma, Italy b Dipartimento di Fisica e Astronomia, Alma Mater Studiorum, Università degli Studi di Bologna, Via Gobetti 93/2, 40129 Bologna, Italy c INAF – Osservatorio di Astrofsica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, I-40129 Bologna, Italy d Center for Relativistic Astrophysics, School of Physics, Georgia Institute of Technology, 837 State Street, Atlanta, GA 30332-0430, United States e Center for Theoretical Astrophysics and Cosmology, Institute for Computational Science, University of Zurich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland f TAPIR, California Institute of Technology, 1200 E. California Blvd., Pasadena, CA 91125, USA g Università degli Studi di Milano-Bicocca, Piazza della Scienza 3, 20126 Milano, Italy h INFN, Sezione di Milano-Bicocca, Piazza della Scienza 3, 20126 Milano, Italy i Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany j Centre for Extragalactic Astronomy, Durham University, South Road, Durham, DH1 3LE, UK k Dipartimento di Fisica e Astronomia, Universitá degli Studi di Firenze, Via. G. Sansone 1, I-50019 Sesto Fiorentino (FI), Italy l INAF - Osservatorio Astrofsico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy m Joint Institute for VLBI ERIC, Oude Hoogeveensedijk 4, 7991 PD, Dwingeloo, The Netherlands n Department of Astronomy, University of Michigan, Ann Arbor, MI 48109, USA o Department of Physics and Astronomy, Vanderbilt University, Nashville, TN 37235, USA p School of Physics and Astronomy and Institute of Gravitational Wave Astronomy, University of Birmingham, Edgbaston, Birmingham B15 2TT, United Kingdom q Universitäts-Sternwarte München, Scheinerstr.1, D-81679 München, Germany r Dipartimento di Matematica e Fisica, Università degli Studi Roma Tre, via della Vasca Navale 84, 00146 Roma, Italy s Sorbonne Universites et CNRS, UMR 7095, Institut d’Astrophysique de Paris, 98 bis bd Arago, F-75014 Paris, France t Konkoly Observatory, Research Centre for Astronomy and Earth Sciences, Konkoly Thege M. út 15–17, H-1121 Budapest, Hungary u MTA-ELTE Extragalactic Astrophysics Research Group, ELTE TTK Pázmány Péter sétány 1/A, H-1117 Budapest, Hungary v Department of Astronomy, Eötvös Loránd University, Pázmány Péter sétány 1/A, H-1117 Budapest, Hungary w Centro de Astrobiología (CSIC-INTA), Departamento de Astrofísica; Camino Bajo del Castillo s/n, Villanueva de la Cañada, E-28692 Madrid, Spain x ESA - European Space Agency. ESTEC, Keplerlaan 1 2201AZ Noordwijk, The Netherlands y Department of Astronomy, Columbia University, New York, NY 10027, USA z Astronomisches Institut, Ruhr-Universität Bochum, Universitätsstrasse 150, 44801 Bochum, Germany A European Southern Observatory, Alonso de Córdova 3107, Vitacura, Casilla 19001, Santiago de Chile, Chile B Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Magrans, E-08193 Barcelona, Spain C ICREA & Institut de Ciències del Cosmos (ICCUB), Universitat de Barcelona (IEEC-UB), Barcelona 08028, Spain D National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, China E SRON, Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA, Utrecht, The Netherlands F Department of Astrophysics/IMAPP, Radboud University, P.O. Box 9010, 6500 GL, Nijmegen, The Netherlands G XMM-Newton SOC, ESAC/ESA, E-28692 Villanueva de la Cañada, Madrid, Spain H Instituto de Astrofísica de Andalucía (IAA-CSIC), E-18008, Granada, Spain I INAF - Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monte Porzio Catone (Roma), Italy ⁎ Corresponding author. E-mail addresses: [email protected] (A. De Rosa), [email protected] (C. Vignali). https://doi.org/10.1016/j.newar.2020.101525 Received 29 July 2019; Received in revised form 10 January 2020; Accepted 28 January 2020 Available online 11 February 2020 1387-6473/ © 2020 Elsevier B.V. All rights reserved. A. De Rosa, et al. New Astronomy Reviews 86 (2019) 101525 ARTICLE INFO ABSTRACT Keywords: The quest for binary and dual supermassive black holes (SMBHs) at the dawn of the multi-messenger era is Galaxies: active compelling. Detecting dual active galactic nuclei (AGN) – active SMBHs at projected separations larger than Galaxies: interactions several parsecs – and binary AGN – probing the scale where SMBHs are bound in a Keplerian binary – is an Galaxies: nuclei observational challenge. The study of AGN pairs (either dual or binary) also represents an overarching theo- Quasars: supermassive black holes retical problem in cosmology and astrophysics. The AGN triggering calls for detailed knowledge of the hydro- gravitational waves dynamical 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 flaments 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 signifcantly advance this fascinating feld. Introduction between ∼ 1 pc and ∼ 100 kpc in dual AGN, while it lies in the pc–sub-pc range in binary systems (pc-scale binaries could fall into ei- 6 9 Supermassive black holes (SMBHs) with mass of ∼ 10 –10 M⊙ are ther category depending on the parameters of the system). ubiquitous in ellipticals, in the bulges of disk galaxies and in at least a A number of studies in diferent wavebands show evidence for a fraction of dwarf galaxies. The tight correlation between the black hole higher fraction of dual AGN in galaxies with a close companion, sug- mass and the bulge stellar velocity dispersion (Ferrarese and Merritt, gesting that galaxy interactions play a role in the AGN triggering pro- 2000; Gebhardt et al., 2000; Kormendy and Ho, 2013) suggests that cess (e.g. Ellison et al., 2011; Koss et al., 2012; Silverman et al., 2011; SMBHs are likely to afect the evolution of the host galaxy over cos- Satyapal et al., 2014; Kocevski et al., 2015; Koss et al., 2018). However, mological time-scales. In contemporary astrophysics, a large variety of there are investigations revealing no enhanced AGN activity in mergers physical phenomena concur to establish the close connection between compared to a matched control sample of inactive galaxies (e.g. the formation and evolution of galaxies and of their central SMBHs (Silk Cisternas et al., 2011; Mechtley et al., 2016). These conficting results and Rees, 1998; Di Matteo et al., 2005). Galaxy mergers may be a way may be a consequence of diferent sample selection criteria (such as through which SMBHs form by direct collapse of gas at the center of merger stage of interacting galaxies and/or AGN luminosity) and ob- protogalaxies (Begelman et al., 2006; Mayer et al., 2010; Mayer and servational biases (e.g., nuclear obscuration and AGN variability). In Bonoli, 2019). Furthermore, there is growing evidence that major light of these uncertainties, the detection and characterization of dual mergers trigger the most luminous AGN (e.g., Treister et al. 2012; Fan and binary SMBHs is fundamental if we want to understand the for- et al. 2016; Goulding et al. 2018), although this result is still debated. mation and accretion history of SMBHs across cosmic ages. Numerical simulations show that galaxy collisions are conducive to A further issue which makes the study of dual and binary AGN one episodes of major gas infows that feed the central SMBH, thus pow- of the forefront topics in modern astrophysics is that these systems are ering accretion and nuclear activity (e.g., Di Matteo et al. 2005). Models the natural precursors of coalescing binary SMBHs, which are strong of structure formation can reproduce the observed large-scale proper- emitters of low-frequency gravitational waves (GWs). Detecting the GW ties of quasars (e.g., their environment and clustering clustering) if their signal from the inspiral, merger and ringdown of binary black holes bright and short-lived active phases are most likely triggered by mer- with LISA, the ESA’s Laser Interferometer Space Antenna (Amaro- gers (see Kaufmann and Haehnelt 2000; Alexander and Hickox 2012 Seoane et al., 2017), will let us unveil the rich population of SMBHs of and references therein). However, not all nuclear activity is triggered 104 7 M forming in the collision of galaxy halos, out to redshifts as by galaxy collisions as SMBH growth can occur also through secular large as z ∼ 20 (Colpi et al., 2019). Note that LISA will detect the processes (Martin et al., 2018; Ricarte et al., 2019) with mergers trig- “light” SMBHs at the low-mass end of their mass distribution. Thus, gering an initial rapid growth phase (McAlpine et al., 2018).