Evidence for Supermassive Black Hole Binaries

TL;DR

This review synthesizes the theoretical expectations and observational evidence for supermassive black hole binaries (SMBHBs), tracing their formation from seed black holes through cosmic growth to the complex gas and stellar dynamics that drive mergers. It contrasts stellar-dynamical hardening in gas-poor environments with gas-driven migration in clumpy, star-forming discs, highlighting stochastic torques, disc breaking, and spin evolution that influence coalescence timescales and gravitational-wave signals. The EM landscape is surveyed from kiloparsec-scale dual AGN to sub-parsec indicators via optical, X-ray, and radio methods, with jet precession and X-ray spin measurements offering indirect SMBHB signatures and merger histories. Gravitational-wave prospects are discussed, including a hints of a common-noise background in pulsar timing arrays and the anticipated birth of multi-band GW astronomy with LISA, which will directly observe SMBHB mergers and constrain their demographics and spin evolution.

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.

Figures (8)

• Figure 1: A summary of the various astrophysical mechanisms that contribute to drive the orbital evolution of massive black hole pairs from large to small scales, adapted from 2023LRR....26....2A under Creative Commons license, https://creativecommons.org/licenses/by/4.0/.
• Figure 2: Directly resolved dual AGN. Top: X-ray maps of the merger remnant NGC 6240 obtained with Chandra. The large map in reddish colours shows the soft 0.5-1.5 keV band. The inset with blueish colours represent the hard 5-8 keV band. The X-ray detection confirms that both nuclei host active AGN and therefore supermassive black holes. Their separation is about 1 kpc in projection. The data was originally published by Komossa2003. Bottom: Radio map at 8 GHz obtained with the Very Long Baseline Array radio telescope of the galaxy 0402+379 Rodriguez06. Two active galactic nuclei are directly detected at 7 pc separation. The weaker nucleus hosts a radio jet. Adopted under Creative Commons license, https://creativecommons.org/licenses/by/4.0/.
• Figure 3: 3D hydrodynamics simulations of precessing jets from Horton23 and Very Large Array radio maps at 5 and 8 GHz of radio galaxies and quasars with jets that show signatures expected from a long-term driven precession, such as the relativistic geodetic spin precession in a binary SMBH (3 examples towards the right, each with their 3C ctalogue number). The three simulations had different precession parameters and form a variety of interactions with the lobe boundary and hotspot structures. Such features can be compared to observed radio galaxies and used to understand the properties of the driving system. For the observed radio maps towards the right, the projected sizes on the sky range between 70 and 330 kpc. Letters mark the occurrence of precession features discussed in more detail in the Sect. \ref{['sec:prec-jets']}: E -- jet detected towards the edge, rather than the middle of the lobe, S -- S-symmetry, C -- jet curvature, R -- ring-like, extended or multiple hotspots. While the simulations on the left do not reproduce any particular system on the right, they do demonstrate that precessing jets may be curved, misaligned with the lobes and have multiple hotspots. Images adopted from Krause19 who find strong indication for jet precession in 24 out of 33 powerful radio galaxies from a complete sample.
• Figure 4: Likely precessing 100 kpc-scale jets and therefore binary black hole candidates from LOFAR. Adopted from 2025arXiv250418518H. The image is a montage of representative precessing sources selected in 2025arXiv250418518H to demonstrate the presence of precession indicators E, C and R discussed in Sect. \ref{['sec:prec-jets']}. There is sometimes a striking similarity with the simulated images, compare, e.g., Fig. \ref{['fig:jetsims']}, left.
• Figure 5: Spin magnitude estimated from X-ray reflection as a function of mass for 51 SMBHs in AGN, compiled from the literature. The figure is adapted from the Reynolds2021 and Bambi2021 spin reviews with the inclusion of: the 13 SMBH spin estimates in low-mass AGN from Mallick2022 (in green) and recent spin constraints for the SMBHs in H 1821+643 Sisk-Reynes2022 and ESO 033-G002 Walton2021. Note some of the estimates from Bambi2021 show a well-defined lower spin bound. We omit the following objects: IRAS 1339+2438 and 4C 74.26 Sisk-Reynes2022, Tons 180, and the sample of type-1 AGN from Mallick2025_unpublished. Error bars in spin/mass correspond to statistical uncertainties at the 68/90 per cent level, respectively. Comparing with predictions from cosmological models, the lower spins at the highest masses is best explained by binary SMBH merging. More measurements are needed to corroborate this result.