A pair of black holes has been observed colliding in the ancient universe for the first time. The observations, by the James Webb Space Telescope, reveal a merger of two galaxies and the monster black holes at their centres when the universe was just 740m years old, about a 20th of its current age.
The discovery that massive mergers appear to have been common in the infant universe could help explain how supermassive black holes like the one at the heart of the Milky Way achieved such tremendous proportions.
Prof Roberto Maiolino, an astrophysicist at the University of Cambridge, and a member of team behind the observations, said: “One problem that we have in cosmology is explaining how these black holes manage to grow so big. In the past we have always talked about gobbling matter very quickly or being born big. Another possibility is that they grow very fast by merging.”
Until now it was not clear whether the merging of galaxies – which is known to have happened – would also result in the black holes at the centres morphing into a single cosmic sinkhole. Recent models have suggested that one of them would be kicked out into space to become a “wandering black hole”.
The latest observations use the Webb telescope’s ability to get to the far reaches of the cosmos and so have provided the first glimpse of galactic mergers in the distant past.
In the process of merging, black holes gobble up huge quantities of matter and also release a lot of energy, and this activity has distinctive spectral features that allow astronomers to identify them. This activity revealed the collision under way in a system called ZS7, with one of the two black holes estimated to have 50m times the mass of the sun.
“The mass of the other black hole is likely similar, although it is much harder to measure because this second black hole is buried in dense gas,” said Maiolino.
Subsequent observations showed that, of black holes detected at this time period, around a third appeared to be in the process of merging. “This could be a real channel for the rapid growth of early black holes,” he said.
Prof Andrew Pontzen, a cosmologist at University College London, who was not involved in the research, said: “One of the major blanks in our cosmic history book is where giant black holes, millions or billions of times the mass of sun, came 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? This new evidence from [the Webb telescope] is indirect, but it helps suggest a major role for black hole collisions.”
Scientists hope in future to be able to make direct measurements of ancient collisions using the next generation of gravitational wave detectors, including the Laser Interferometer Space Antenna (Lisa) mission, which was recently approved by the European Space Agency.
The findings are published in the Monthly Notices of the Royal Astronomical Society.
