A rare collision between two supermassive black holes (SMBH) appears to have sent the resulting merger speeding through the universe, making it one of the fastest-moving black holes we have ever seen.
Astronomers have long puzzled over how the gargantuan black holes at the centres of galaxies can grow to be so large. One possible route is for smaller 鈥� but still extremely massive 鈥� black holes to merge together, but there has been little direct evidence of this happening.
Now, at Johns Hopkins University in Maryland and his colleagues say they have found evidence of a supermassive black hole at the centre of a galaxy called 3C 186 that has been kicked out of its galactic nesting spot, rocketing away at more than a thousand kilometres per second.
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Astronomers had previously observed the galaxy using the Hubble Space Telescope and found that its quasar, the powerful light given off by its central black hole, was out of place. By studying the distribution of stars in the galaxy, they realised the SMBH was actually around 33,000 light years away from the galactic centre where it should be. This implied the black hole had been forced out of line by some unknown event, like a merger.
Chiaberge and his team used the Very Large Telescope in Chile and the Subaru Telescope in Hawaii to analyse the light given off by the black hole more carefully. They found the light coming from its accretion disk, where matter orbiting the black hole is violently heated, was blueshifted relative to the galaxy as a whole, an effect from general relativity that changes the colour of light when something is travelling extremely quickly towards the observer. However, the gas in the surrounding region showed less sign of being blueshifted, implying the black hole is moving relatively more swiftly than the rest of the galaxy.
The researchers argue the most probable explanation is two galaxies combined and their central SMBHs smashed together to form a single larger one. This merger would have produced ripples in space-time called gravitational waves that travelled out in one direction, while the newly formed black hole recoiled the other way.
鈥淭he evidence for a recoil kick appears strong and, while there is never certainty in astrophysics, this is convincing,鈥� says at the University of Surrey, UK.
at the University of California, Berkeley says while the source is one of the better candidates for a supermassive black hole merger, it is less convincing because of the difficulty in interpreting the light coming from the region around a SMBH, called the active galactic nucleus (AGN). It is not unusual for AGNs to appear to be moving quickly even if they haven鈥檛 merged, so we can鈥檛 take that as definitive evidence for a recoiling SMBH scenario, he says. 鈥淢uch more careful physical modelling of the broad-line region is required to become confident in the recoil hypothesis.鈥�
arXiv
Article amended on 18 March 2025
We have clarified the blueshift of the light from the black hole
