By analysing data on around 100,000 quasars, or extremely luminous galactic nuclei, researchers at Eötvös Loránd University (ELTE) have found statistical evidence that some supermassive black holes at the centres of galaxies are ejected from their positions after forming. The relationship now identified in the data was predicted ten years ago by members of the same research group, Peter Raffai and Zsolt Frei, together with Zoltán Haiman. The present study was led by Bence Bécsy, who graduated from ELTE and now works at the University of Birmingham. The findings have been published in the journal Monthly Notices of the Royal Astronomical Society .
At the centre of every large galaxy lies a supermassive black hole, whose mass can be millions or even billions of times that of the Sun. When two galaxies merge, their central black holes also approach one another, form a binary system, emit gravitational waves and eventually merge themselves, leaving behind an even larger black hole.
In the final moments of the merger, extremely powerful gravitational waves are generated. If these waves are not emitted equally in all directions, the remaining black hole receives a powerful “kick”, similar to the recoil of a gun when a bullet is fired. The velocity of the black hole propelled in this way can reach several million kilometres per hour, displacing it from the centre of its galaxy.
However, a kicked black hole does not simply disappear. If the kick is not strong enough to eject it from the galaxy entirely, the black hole falls back, speeds through the galactic centre, slows down on the other side and turns back. It then oscillates around the galactic nucleus for a long time, like a pendulum. And just as a pendulum moves fastest at the lowest point of its swing, the black hole also moves fastest as it passes through the centre of the galaxy.
According to general relativity, such a recoil must occur, but the phenomenon has not yet been unambiguously demonstrated. Astronomers have identified several individual candidates, but alternative explanations could not be ruled out in those cases. The present study therefore did not search for a single unusual object. Instead, it investigated whether the statistical signature of ejected black holes could be detected across the data of tens of thousands of quasars.
Some supermassive black holes are active: they consume large amounts of gas from their surroundings and, in the process, emit extremely intense radiation. These luminous, active galactic nuclei are known as quasars. Their light contains two kinds of information that the researchers needed.
The first is the velocity of the black hole. Gas located close to the black hole moves with it, whereas gas somewhat farther away does not. The light emitted by the two regions is therefore shifted slightly relative to one another, much as the siren of an approaching or receding ambulance sounds higher or lower in pitch than that of a stationary ambulance. This small shift makes it possible to determine how fast the black hole is moving relative to its host galaxy.
The second piece of information concerns the amount of dust. Dust surrounding the black hole makes the quasar's light appear redder, much as the setting Sun looks red through a thick layer of atmosphere. If a quasar is redder than comparable objects, this indicates that more dust is obscuring it.
In 2016, Peter Raffai, Zoltán Haiman and Zsolt Frei used computer models to investigate what we would observe if, in some quasars, the black hole were an ejected object oscillating around the galactic nucleus. They concluded that there should be a relationship between velocity and dust content: the black hole moves fastest at the centre of the galaxy, where the dust is also densest. Quasars containing faster-moving black holes should therefore, on average, be obscured by more dust. The authors already pointed out at the time that this relationship could be investigated using real data from large quasar catalogues.
“We have now detected in real observational data the relationship we predicted years ago. It is difficult to think of a better example of why being a researcher is so rewarding. Our findings may help us understand the processes taking place in galactic nuclei and the role played by Einstein's gravitational waves in these processes,” said Peter Raffai , Associate Professor at ELTE and lead author of the 2016 study.
The study testing the theory was carried out by Bence Bécsy and his colleagues. The researchers analysed approximately 100,000 quasars from the quasar catalogue of the Sloan Digital Sky Survey, selecting objects for which sufficiently high-quality data were available. For each quasar, they determined the black hole's velocity and the quasar's redness, then investigated whether the two were related.
The relationship predicted ten years ago emerged from the data: quasars containing faster-moving black holes are, on average, redder, meaning that more dust obscures them. Natural differences between individual quasars make the relationship difficult to identify in any single object, but across this enormous sample it is statistically very strong. The most striking difference appears among the dustiest quasars: the proportion of heavily dust-obscured objects is three times higher among quasars containing the fastest-moving black holes than among those containing the slowest-moving ones.
“It is very difficult to prove that the black hole in a single quasar has been ejected, because other phenomena can produce similar signatures. What is new about our research is that, rather than looking for individual candidates, we searched statistically for traces of an entire population. This enabled us to identify a relationship consistent with theoretical predictions,” said Bence Bécsy , lead author of the study.
The researchers also carried out several validation tests. The result remained consistent under different data-selection and measurement procedures. However, when they compared only the light from gas clouds that do not follow the black hole's motion, the positive correlation disappeared.
The most natural explanation for the observed relationship is that some of the quasars in the sample contain supermassive black holes ejected following an earlier merger. If this interpretation is correct, the strength of the signal also suggests that two black holes merged relatively recently in roughly half of the quasars studied. The authors emphasise, however, that this estimate remains uncertain and that other physical processes cannot be completely ruled out. Further observations and more detailed models will be needed.
The lead author of the study, Bence Bécsy, graduated in physics from Eötvös Loránd University, where he completed his thesis under the supervision of Peter Raffai, before continuing his scientific career abroad. He is currently an Assistant Professor at the University of Birmingham in England. He is also a member of the NANOGrav collaboration, which uses pulsar signals to investigate very low-frequency gravitational waves from supermassive black hole binaries. These waves were first detected in 2023.
The ELTE-affiliated authors of the study are Associate Professor Peter Raffai and Professor Zsolt Frei, members of ELTE's Institute of Physics and Astronomy and the HUN-REN–ELTE Extragalactic Astrophysics Research Group. A co-author is Zoltán Haiman, Professor at the Institute of Science and Technology Austria and Columbia University, who also co-authored the 2016 theoretical study. Another co-author is Andor Budai, who previously pursued his doctoral studies at ELTE under Peter Raffai's supervision.
The result is the first observational test using a large sample of a prediction formulated at ELTE ten years ago. If further research confirms this interpretation, the method could also be used to investigate how frequently supermassive black holes merge, how fast they are ejected and what kind of environments surround them.
Research into ejected black holes also contributes to preparations for the largest gravitational-wave instrument of the future. In 2035, the European Space Agency plans to launch the LISA space observatory: three spacecraft orbiting in formation, 2.5 million kilometres apart, which will directly detect gravitational waves from supermassive black hole mergers. These are precisely the events whose traces the researchers have now identified in quasar light. ELTE researchers have been involved in the scientific preparations for LISA since 2006, and from 2026 they will continue this work within a new framework. Following an August decision by the National Research, Development and Innovation Office, the Kármán Tódor Space Science Mission-Driven National Laboratory will be established at ELTE, with the Konkoly Observatory of the HUN-REN Research Centre for Astronomy and Earth Sciences as a collaborating institution. The four-year programme has a budget of HUF 1.12 billion and is led by principal investigator Zsolt Frei. Its gravitational-wave research will focus on developing waveform models, which will be essential for extracting information about black hole masses, distances and fates from LISA's signals. The present findings thus help lay the groundwork for a question that LISA may provide a definitive answer to in the coming decade.
“If the signal is indeed caused by ejected black holes, then two supermassive black holes merged relatively recently in a significant proportion of quasars, meaning that these events are common. This directly determines how many such events LISA may be able to detect. We put this idea forward at ELTE ten years ago, and now we have tested it ourselves, led by Bence Bécsy, who graduated from our university. Few research groups are fortunate enough to maintain this kind of continuity,” said Professor Zsolt Frei , head of the HUN-REN–ELTE Extragalactic Astrophysics Research Group.
The newly published study:
Bécsy, B., Raffai, P., Haiman, Z., Budai, A. & Frei, Z.: „Statistical evidence for massive black hole recoils in active galactic nuclei”, Monthly Notices of the Royal Astronomical Society 550 (4), stag1367 (2026). The article is freely available; a preprint is available on arXiv: arXiv:2605.04781 .
The theoretical precursor:
Funding:
The work of the ELTE researchers involved in this study was supported by the HUN-REN Hungarian Research Network and by the National Research, Development and Innovation Office through its TKP2021-NKTA-64 excellence grant.
Monthly Notices of the Royal Astronomical Society
Statistical evidence for massive black hole recoils in active galactic nuclei
4-Aug-2026