This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: By analyzing 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 centers of galaxies are ejected from their positions after forming. The relationship now identified in the data was predicted 10 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 center 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 kilometers per hour, displacing it from the center 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 center, 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 center 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.
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