James Webb Telescope Detects Earliest Known Black Hole Merger, Just 740 Million Years After the Big Bang

The new observations could help explain how black holes became so massive in the early universe

An image of galaxies and stars observed by the Webb Telescope
The environment surrounding the galaxy system where the two black holes are merging. The event is occurring in the distant universe, around 13 billion years ago. ESA / Webb, NASA, CSA, J. Dunlop, D. Magee, P. G. Pérez-González, H. Übler, R. Maiolino, et al.

Astronomers have discovered the earliest evidence of two galaxies and their massive black holes merging, just 740 million years after the Big Bang.

The findings, published Thursday in the journal Monthly Notices of the Royal Astronomical Society, contribute to scientists’ understanding of how black holes grew to be so large in the universe’s early days. Researchers used the James Webb Space Telescope to peer 13 billion years back in time and observe the merger.

Scientists are unsure as to where giant black holes—with masses millions or billions of times that of our sun—come from.

“Do they somehow get born big, or do they have to be built from initially smaller black holes that smash together to form the giants?” Andrew Pontzen, a cosmologist at University College London who did not contribute to the findings, says to the Guardian’s Hannah Devlin. “This new evidence from [the Webb telescope] is indirect, but it helps suggest a major role for black hole collisions.”

“Our findings suggest that merging is an important route through which black holes can rapidly grow, even at cosmic dawn,” Hannah Übler, a researcher at the University of Cambridge in England, says in a statement from the European Space Agency. “Together with other Webb findings of active, massive black holes in the distant universe, our results also show that massive black holes have been shaping the evolution of galaxies from the very beginning.”

three images side by side, each getting progressively closer to the system ZS7, where the two merging black holes were seen.
The location of ZS7 in Webb's image, where astronomers found evidence of the earliest known merger of black holes. ESA / Webb, NASA, CSA, J. Dunlop, H. Übler, R. Maiolino, et al.

Black holes are enormous collections of matter packed into relatively small spaces. They’re so dense that nothing, including light, can escape their immense gravitational pulls. Nearly every large galaxy has a supermassive black hole at its center.

With recent studies using the Webb telescope, researchers have found a large number of black holes that attained incredible masses in the first billion years of the universe, per the new paper. In January, astronomers discovered the oldest known black hole—and it was much larger than they expected. This meant it either started out larger than scientists thought was possible, or it grew surprisingly quickly.

Possible sources of black hole growth in the early universe include the collapse of primordial clouds, remnants of early giant stars and the merging of stars and black holes, per the study. Astronomers had also suggested black holes could merge and become more massive, but previous findings about that are “tentative and indirect,” the paper’s authors write.

For the new study, the researchers looked at a system called ZS7. They identified the two black holes based on evidence for very dense, fast-moving gas nearby, as well as hot and highly ionized gas illuminated by radiation that is usually produced by black holes while they’re accumulating matter, per the ESA’s statement.

The researchers measured the mass of one of the black holes as 50 million times that of our sun. They think the other black hole has a similar mass but were unable to measure it, because it was blocked by dense gas.

In the future, gravitational wave observatories, like the Laser Interferometer Space Antenna that is expected to launch to space in the mid-2030s, could allow scientists to measure the ripples in spacetime emanating from the merging of these two black holes and other mergers from long ago.

“Webb’s results are telling us that lighter systems detectable by LISA should be far more frequent than previously assumed,” Nora Luetzgendorf, LISA lead project scientist with the European Space Agency, says in the statement. “This is just the tip of the iceberg.”

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