A map of the distant galaxy, showing the locations of the three massive black holes, indicated by black circles (not to scale). The most massive and least massive black holes are located in the galaxy centre, and a third black hole is in the galaxy outskirts

Hannah Übler

Three massive black holes have been discovered in a galaxy located in the early universe, 12.5 billion light years away. This has left astronomers surprised that such a complex system could have formed so soon after the big bang.

“This system is in the very, very early universe,” says Dougal Dobie at the University of Sydney, who wasn’t involved in the discovery. “So, how have we ended up in a situation where these three supermassive black holes have formed and come close together in only a billion years or so since the big bang?”

In December 2024, Hannah Übler at the Max-Planck Institute for Extraterrestrial Physics in Germany and her colleagues were observing a galaxy known as J0148-4214 through the James Webb Space Telescope when they saw something strange.

At 12.5 billion years’ distance, the team observed the galaxy as it looked around 1.2 billion years after the big bang, says Übler. “We [also] quickly noticed that the central emission profile of the galaxy had a peculiar shape,” she says.

An analysis revealed two separate regions of high-velocity hydrogen gas in the centre of the galaxy, indicating two so-called accreting black holes that were separated by about 620 light years – a hop, skip and a jump in cosmic terms.

An accreting black hole is a super dense area of space, so compressed that light can’t escape, but that continuously drags in surrounding gas, dust and even stars. Our own galaxy, the Milky Way, has just one massive black hole at the core of its spiral arms.

“During examination of the whole galaxy, we also noticed another region in the galaxy outskirts with high-velocity hydrogen gas, indicating another, third growing black hole,” says Übler. This one is more than 5500 light years away from the other two.

“We have explored several alternatives ranging from a single black hole to supernovae, shocks, winds and massive stars, but these were all unsatisfactory or incompatible based on our data and previous observations of this system,” she says.

Until now, there was no evidence that massive black hole triplets existed within the first 1.2 billion years of cosmic history, says Übler. “This discovery shows us that processes in the early universe were efficient at bringing massive black holes together,” she says. “It also suggests that black hole merging may have been an additional, fast route for their rapid growth in the early universe.”

Caroline Foster at the University of New South Wales in Sydney, Australia, says that based on the information the researchers have presented, she agrees with their conclusion. “The fingerprints look like black holes and the ‘shape’ is best explained with two separate ones,” she says. “The third black hole is better separated, so it is a much cleaner result. Maybe it is falling into the galaxy for the first time, or maybe it has been ejected by interactions with the other two.”

Having three black holes in close vicinity and in a single system suggests there has recently been a merger of separate galactic entities, says Foster. But she says other interpretations may arise in the future, as more observations of the galaxy are made.

Dobie says that a triple black hole system “seems to be the most promising interpretation of the data that they’re seeing”. He also suspects that as more telescopes come online and technology advances, more such galaxies will be found.