Could We Detect the Merger of Stellar-Mass and Supermassive Black Holes?

We’ve detected gravitational waves from mergers of compact objects like stellar-mass black holes, and we’ve found promising evidence for the spacetime disturbances from binary supermassive black holes. But what about when these two mass scales meet — could we detect the merger of a stellar-mass black hole with a supermassive black hole?

Stellar-Mass and Supermassive

Many galaxies host a central supermassive black hole, which may have the opportunity to consume stellar-mass black holes from its surroundings. Based on theoretical calculations, it’s likely fairly rare for a stellar-mass black hole to merge with a supermassive black hole, with each galaxy experiencing just a few dozen of these events every billion years.

infographic showing the relative frequencies of gravitational waves from different sources

Infographic showing the typical frequency ranges of the gravitational waves produced by different sources. Click to enlarge. [ESA]

Surprisingly, adding another supermassive black hole into the mix may greatly increase the odds of such an interaction. When stellar-mass black holes encounter a supermassive black hole binary, the likelihood of a merger is boosted up to hundreds of thousands of events per galaxy per billion years. The gravitational waves from this type of merger are too low frequency to be detected with our current observatories, but a recent research article has explored the possibility of detecting gravitational waves from these encounters in the not-too-distant future.

Simulating Gravitational Waves

Smadar Naoz (University of California, Los Angeles) and Zoltán Haiman (Columbia University) simulated the gravitational waves that would result from a stellar-mass black hole spiraling in to merge with one member of a supermassive black hole binary. This type of merger is called an extreme-mass-ratio inspiral. First, Naoz and Haiman estimated the number of extreme-mass-ratio inspirals as a function of the mass of the black holes in the binary system. Perhaps counterintuitively, stellar-mass black holes are much more likely to merge with the less massive black hole in a binary system, thanks to gravitational perturbations.

Plot of gravitational wave strain versus frequency

Predicted gravitational wave amplitude as a function of frequency for resolved (purple lines) and unresolved (grey lines) systems, compared to LISA’s estimated sensitivity. Click to enlarge. [Naoz & Haiman 2023]

The team then calculated the amplitude of the gravitational waves produced in each merger and found that future observatories should be able to detect these events. They focused on the Laser Interferometer Space Antenna (LISA) — a proposed space-based gravitational wave observatory that would consist of three spacecraft trailing Earth in its orbit — which should detect individual extreme-mass-ratio inspirals as well as a background signal composed of thousands of events too faint to be detected individually. During the proposed 4-year LISA mission, the observatory could detect hundreds of individual sources.

illustration of the LISA gravitational wave observatory

Illustration of the three LISA spacecraft trailing behind Earth. [NASA/JPL-Caltech/NASAEA/ESA/CXC/STScl/GSFCSVS/S.Barke; CC BY 4.0]

Observing gravitational waves from a stellar-mass black hole as it spirals toward a supermassive black hole can help us understand many aspects of how supermassive black holes grow and merge. In particular, these observations may help us put a number on how many companions a supermassive black hole is likely to have; do these behemoths mostly fly solo, or are pairs, triples, or quartets more likely? Hopefully, it’s just a matter of time before LISA is in place in its berth in space — the planned launch date is 2037 — and ready to open a new window onto gravitational waves.


“The Enhanced Population of Extreme Mass-Ratio Inspirals in the LISA Band from Supermassive Black Hole Binaries,” Smadar Naoz and Zoltán Haiman 2023 ApJL 955 L27. doi:10.3847/2041-8213/acf8c9