Evidence for Supermassive Black Hole Binaries
Abstract
We review the state of the evidence for the existence and observational appearance of supermassive black hole binaries. Such objects are expected from standard hierarchical galaxy evolution to form after two galaxies, each containing a supermassive black hole, have merged, in the centre of the merger remnant. A complex interaction is predicted to take place with stars and gas in the host galaxy, leading to observable signatures in weakly as well as actively accreting phases. Direct observational evidence is available and shows examples of dual active galactic nuclei from kpc scales down to parsec scales. Signatures of possibly closer supermassive black hole binaries may be seen in jetted black holes. The interaction with stars and gas in a galaxy significantly affects the hardening of the binary and hence contributes to uncertainties of the expected gravitational wave signal. The Laser Interferometer Space Antenna (LISA) should in the future detect actual mergers. Before the launch of LISA, pulsar timing arrays may have the best chance to detect a gravitational wave signal from supermassive black hole binaries. The first signs of the combined background of inspiralling objects might have been seen already.
Cite
@article{arxiv.2510.07534,
title = {Evidence for Supermassive Black Hole Binaries},
author = {Martin G. H. Krause and Martin A. Bourne and Silke Britzen and Adi Foord and Jenny E. Greene and Melanie Habouzit and Maya A. Horton and Lucio Mayer and Hannah Middleton and Rebecca Nealon and Julia M. Sisk-Reynés and Christopher S. Reynolds and Debora Sijacki},
journal= {arXiv preprint arXiv:2510.07534},
year = {2025}
}
Comments
20 pages, 8 figures, PASA, accepted, minor updates (one afil., a clarification in Sect. 7 and a few references added)