Black holes are fascinating regions in space where gravity is so strong that nothing, even light, can escape them. The largest known black holes, called supermassive black holes, contain millions to billions of times the mass of the sun and are typically located at the centers of galaxies.
About three years ago, astronomers observed a “runaway” supermassive black hole that appeared to be traveling through space at almost 1,000 km/s (620 miles per second). This black hole was detected by both the James Webb Space Telescope (JWST) and the Hubble Space Telescope (HST), two of the largest space observatories built to date.
Researchers at the Kavli Institute for Theoretical Physics (KITP), University of California-Santa Barbara (UCSB) and the University of Texas at Austin recently tried to shed more light on the origins of this traveling black hole, which is known as RBH-1.
Their paper, published in Physical Review Letters, suggests that the black hole may have been ejected into space by the merger of two supermassive black holes.
“Even though recoiling supermassive black holes have previously been proposed, RBH-1 is the first convincing example of a supermassive black hole that appears to have been ejected from the center of its host galaxy, likely as the result of a powerful gravitational-wave recoil following the merger of two supermassive black holes,” Tousif Islam, first author of the paper, told Phys.org.
“It is a spectacular confirmation of one of the most fascinating predictions of Einstein’s general theory of relativity. For us, that discovery naturally led to the next question.”
Reconstructing the merger preceding RBH-1
In an earlier paper, Islam and other researchers showed that the black hole left behind after a black hole merger could be expelled into space with a high recoil velocity, or kick, of up to 5,000 km/s (3,100 miles per second). A black hole merger could thus potentially explain the behavior of the rapidly moving RBH-1 black hole observed by JWST and HST.
In collaboration with Tejaswi Venumadhav and Digvijay Wadekar, Islam set out to reconstruct the black hole merger that might have prompted the large gravitational-wave recoil (i.e., “kick”) that caused RBH-1 to travel at such high speed through space. To do this, they relied on a combination of theory-based simulations and theoretical frameworks.
“We asked: Could the observed properties of RBH-1 reveal the masses and spins of the two supermassive black holes that merged around 70 million years ago?” said Islam. “More broadly, could we use a runaway black hole as a window into the history of a galaxy merger that we can no longer observe directly? The primary objective of our work was to answer these questions.”
Most black hole mergers produce relatively modest recoils that would be too small to eject the emerging, merged black hole from its galaxy. To reconstruct the merger that would have prompted RBH-1 to move so quickly through space, the researchers started by estimating the speed at which it was traveling after it was ejected from its galaxy.
“As gravitational-wave astronomy experts, we approached the problem like a forensic investigation,” explained Islam. “The key clue was the extraordinary speed of RBH-1—nearly 1,000 km/s (620 miles per second)—as inferred from HST and JWST observations. Our question was simple: What kind of binary supermassive black hole merger could have launched a black hole at such an incredible speed?
“To answer this, we combined the observational measurements with theoretical models of binary black hole mergers developed from highly accurate numerical relativity simulations, which solve Einstein’s equations on supercomputers, together with black hole perturbation theory.”
The theoretical models that the team relied on predict the contribution of the masses and spins of two merging black holes to the mass, spin and recoil velocity of the remnant black hole arising from the merger. The researchers then compared millions of possible progenitor binary black hole configurations with the observed properties of RBH-1.
“This allowed us to identify combinations that are most consistent with the data and place quantitative constraints on the progenitor system,” said Islam. “We showed that the merger most likely involved two supermassive black holes with a mass ratio below about 6:1, with the larger black hole spinning rapidly, and that the binary was likely precessing before it merged.”
Informing future gravitational-wave research
This study shows that supermassive black hole merger reconstructions could be used to uncover the physical properties that could have led to the emergence of specific supermassive black holes. Employing this approach, Islam, Venumadhav and Wadekar were able to infer the properties of the two supermassive black holes that merged to produce RBH-1 and gain insight into the galaxy merger that may have brought them together.
“More broadly, our work shows how astronomical observations and theoretical models based on Einstein’s general relativity can be combined to ‘rewind the clock’ and uncover the history of remarkable cosmic events,” said Islam.
“Looking ahead, we hope this work opens a new avenue for studying supermassive black hole mergers. As JWST, the Nancy Grace Roman Space Telescope and future observatories discover more recoiling supermassive black holes, the same approach can be used to reconstruct their merger histories and better understand how galaxies and their central black holes evolve together.”
In the future, this research team’s efforts could complement direct gravitational-wave observations of supermassive black hole mergers collected by the Laser Interferometer Space Antenna (LISA). LISA is a space-based gravitational-wave observatory being built by the European Space Agency (ESA) in partnership with NASA.
The researchers are now working to further enhance the modeling of binary black hole mergers. Specifically, they plan to develop more accurate theoretical models of binary black hole mergers based on Einstein’s theory of general relativity.
“We also hope to apply the methods we used to future discoveries of recoiling supermassive black holes from JWST, the Nancy Grace Roman Space Telescope and other observatories,” added Islam.
“As the number of observed systems grows, we will be able to reconstruct their merger histories and gain a deeper understanding of how supermassive black holes and their host galaxies evolve together. Ultimately, we hope these efforts will complement future gravitational-wave observations of supermassive black hole mergers by LISA, providing a more complete picture of these extraordinary events.”
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Publication details
Tousif Islam et al, Progenitor of the Recoiling Supermassive Black Hole RBH-1 Identified Using HST and JWST Imaging, Physical Review Letters (2026). DOI: 10.1103/fm3n-sy3f. On arXiv: arxiv.org/abs/2601.18986
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Runaway supermassive black hole RBH-1 offers new way to reconstruct ancient merger (2026, August 5)
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