Astronomers have found supermassive black holes with masses of millions to billions times that of the Sun in most massive galaxies in the local Universe, including in our Milky Way galaxy. These black holes have likely had a major impact on the evolution of the galaxies they reside in. However, scientists still don’t fully understand how these objects grew to become so massive.

The finding of gargantuan black holes already in place in the first billion years after the Big Bang indicates that such growth must have happened very rapidly, and very early. Now, the James Webb Space Telescope is shedding new light on the growth of black holes in the early Universe.

The new Webb observations have provided evidence for an ongoing merger of two galaxies and their massive black holes when the Universe was just 740 million years old. The system is known as ZS7.

Massive black holes that are actively accreting matter have distinctive spectrographic features that allow astronomers to identify them. For very distant galaxies, like those in this study, these signatures are inaccessible from the ground and can only be seen with Webb.

“We found evidence for very dense gas with fast motions in the vicinity of the black hole, as well as hot and highly ionised gas illuminated by the energetic radiation typically produced by black holes in their accretion episodes,” said lead author Dr Hannah Übler of Cambridge’s Cavendish Laboratory and Kavli Institute for Cosmology. “Thanks to the unprecedented sharpness of its imaging capabilities, Webb also allowed our team to spatially separate the two black holes.”

The team found that one of the two black holes has a mass that is 50 million 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 team member Professor Roberto Maiolino, also from the Kavli Institute.

“Our findings suggest that merging is an important route through which black holes can rapidly grow, even at cosmic dawn,” said Übler. “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.”

The team notes that once the two black holes merge, they will also generate gravitational waves. Events like this will be detectable with the next generation of gravitational wave observatories, such as the upcoming Laser Interferometer Space Antenna (LISA) mission, which was recently approved by the European Space Agency and will be the first space-based observatory dedicated to studying gravitational waves.

This discovery was from observations made as part of the Galaxy Assembly with NIRSpec Integral Field Spectroscopy programme. The team has recently been awarded a new Large Programme in Webb’s Cycle 3 of observations, to study in detail the relationship between massive black holes and their host galaxies in the first billion years. An important component of this programme will be to systematically search for and characterise black hole mergers. This effort will determine the rate at which black hole merging occurs at early cosmic epochs and will assess the role of merging in the early growth of black holes and the rate at which gravitational waves are produced from the dawn of time.

These results have been published in the Monthly Notices of the Royal Astronomical Society.

Reference: Hannah Übler et al. ‘GA-NIFS: JWST discovers an offset AGN 740 million years after the big bang’ Monthly Notices of the Royal Astronomical Society (2024). DOI: 10.1093/mnras/stae943

Adapted from a press release by the European Space Agency.