Quasar survey finds statistical evidence of recoiling supermassive black holes

An analysis of approximately 100,000 quasars has found a statistically strong relationship between the velocity of a supermassive black hole and the amount of dust obscuring its host active galactic nucleus. The result is consistent with some black holes having been ejected from the centres of their galaxies after mergers, although it does not establish the history of any individual object. Researchers at Eötvös Loránd University in Hungary and their collaborators used data from the Sloan Digital Sky Survey to test a relationship predicted by members of the same group a decade earlier. Faster-moving black holes were, on average, associated with redder quasars, a pattern expected if recoiling black holes pass through dust-rich regions near their galactic centres.

The study examined a population rather than searching for a single unusual quasar. That distinction addresses a central difficulty in astronomy: several individual candidates for recoiling black holes have been proposed, but observations of one object can have alternative explanations. The new analysis instead asked whether the signature predicted for an entire population appears consistently across a very large catalogue. The findings were published in the peer-reviewed journal Monthly Notices of the Royal Astronomical Society, and the authors describe the work as the first observational test of a prediction formulated at ELTE in 2016.

Supermassive black holes at the centres of large galaxies can have masses of millions or billions of times the Sun’s mass. When two galaxies merge, their central black holes approach one another, form a binary system, emit gravitational waves and eventually merge into a larger remnant. During the final moments of that coalescence, gravitational waves may be emitted unequally in different directions. The resulting recoil can propel the remaining black hole at several million kilometres per hour. If the recoil is too weak to remove the black hole from the galaxy altogether, it can enter a long, pendulum-like orbit around the galactic nucleus, moving rapidly as it crosses the centre before slowing, turning back and eventually returning again.

The quasar sample provided two measurements needed to test that model. The first was the black hole’s velocity relative to its host galaxy. Gas close to the black hole moves with it, while gas at a somewhat greater distance does not move in exactly the same way. The light from these regions is therefore shifted slightly relative to one another, allowing the black hole’s motion to be inferred. The information is analogous to the change in the perceived pitch of a siren as an ambulance approaches or recedes: small shifts in the emitted light reveal whether the central source is moving relative to the surrounding system.

The second measurement concerned dust. Dust surrounding a black hole makes a quasar’s light appear redder than that of comparable objects, providing an indication that more dust is obscuring the active nucleus. In a computer model published in 2016, Peter Raffai, Zoltán Haiman and Zsolt Frei predicted that quasars containing recoiling black holes should show a velocity–dust relationship. According to the model, a recoiling black hole moves fastest at the galactic centre, where the dust is also densest. Quasars with faster-moving black holes should therefore, on average, be redder because they contain more obscuring dust.

For the observational test, the researchers selected approximately 100,000 quasars from the Sloan Digital Sky Survey catalogue for which sufficiently high-quality data were available. They determined the black-hole velocity and the redness of each quasar, then tested whether the two measurements varied together. The predicted relationship emerged: quasars containing faster-moving black holes were, on average, redder and therefore more dust-obscured. Natural differences among individual quasars make the pattern difficult to recognize in any single system, but its statistical strength becomes apparent across the enormous sample.

The sharpest contrast occurred among the dustiest quasars. The proportion of heavily dust-obscured objects was three times higher among quasars containing the fastest-moving black holes than among those containing the slowest-moving black holes. Bence Bécsy, lead author of the new study and an assistant professor at the University of Birmingham, said the central advance was the search for statistical traces of a population rather than proof based on one candidate. “It is very difficult to prove that the black hole in a single quasar has been ejected, because other phenomena can produce similar signatures,” Bécsy said. “What is new about our research is that, rather than looking for individual candidates, we searched statistically for traces of an entire population.”

Several validation tests examined whether the result depended on the team’s choices about data selection or measurement. The relationship remained consistent when the researchers varied those procedures. It disappeared, however, when they used only light from gas clouds that do not follow the black hole’s motion. The result therefore depended specifically on the emission from the region that moves with the black hole, as expected if the measured velocity offset is encoding black-hole recoil. The authors nevertheless cautioned that other physical processes cannot be completely excluded, and said that further observations and more detailed models would be needed.

The most natural explanation for the observed relationship is that some quasars in the sample contain supermassive black holes displaced by recoil following an earlier merger. If that interpretation is correct, the strength of the statistical signal suggests that two black holes merged relatively recently in roughly half of the quasars studied. The authors emphasised that this estimate remains uncertain. It is an inference based on the strength of the population-level relationship, not a direct count of mergers or proof that every displaced black hole in the catalogue arose from a recent coalescence. Other explanations for part of the signal have not been completely ruled out, so the result does not yet provide a definitive merger rate.

The findings may nevertheless inform the future of gravitational-wave astronomy. The European Space Agency plans to launch the LISA space observatory in 2035, with three spacecraft orbiting in formation and separated by 2.5 million kilometres. LISA is intended to detect gravitational waves directly from supermassive black-hole mergers, the events that can produce the recoils examined in the quasar data. If the observed relationship is caused by ejected black holes, it indicates that such recent mergers occur in a significant proportion of quasars and therefore helps determine how many comparable events LISA may detect. ELTE researchers have worked on LISA preparations since 2006, including future work on waveform models intended to extract information about black-hole masses, distances and final fates from the signals.

The project also demonstrates a direct link between an earlier theoretical prediction and a large observational test. Raffai, Haiman and Frei had used computer models in 2016 to show how the signatures of oscillating, recoiling black holes might be detected statistically in active galactic nuclei. Ten years later, Bécsy led the test with Raffai, Haiman, Andor Budai and Frei, using real quasar observations. Bécsy had completed his physics degree and doctoral thesis at ELTE under Raffai’s supervision before moving to Birmingham. “We have now detected in real observational data the relationship we predicted years ago,” Raffai said. The result provides strong statistical support for the predicted population signature while leaving the histories of individual black holes, the precise frequency of mergers and the implications for future gravitational-wave detections to be tested through additional observations and modelling.

Subject of Research: Statistical detection of supermassive black-hole recoil in a large quasar sample

Article Title: Predicted a decade ago, now discovered

Article References: Predicted a decade ago, now discovered. Original publication (Bibliographic details unavailable.)

Image Credits: AI Generated

DOI: Not provided

Keywords: supermassive black holes, quasars, galaxy mergers, black-hole recoil, gravitational waves, Sloan Digital Sky Survey, dust obscuration, active galactic nuclei, LISA, astrophysics, Predicted, decade