Webb Detects Ongoing Merger of Two Supermassive Black Holes in Early Universe

Astronomers using the NASA/ESA/CSA James Webb Space Telescope have found evidence for an ongoing merger of two galaxies and their central black holes — a galaxy system dubbed ZS7 — when the Universe was just 740 million years old.

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 ionized gas illuminated by the energetic radiation typically produced by black holes in their accretion episodes,” said Dr. Hannah Übler, an astronomer at the University of Cambridge.

“Thanks to the unprecedented sharpness of its imaging capabilities, Webb also allowed our team to spatially separate the two black holes.”

The astronomers 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 Dr. Roberto Maiolino, an astronomer at the University of Cambridge and University College London.

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

“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 stellar mass of the system we studied is similar to that of our neighbor the Large Magellanic Cloud,” said Dr. Pablo G. Pérez-González, an astronomer at the Centro de Astrobiología.

“We can try to imagine how the evolution of merging galaxies could be affected if each galaxy had one supermassive black hole as large or larger than the one we have in the Milky Way.”

The researchers also note 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 ESA’s upcoming Laser Interferometer Space Antenna (LISA) mission.

“Webb’s results are telling us that lighter systems detectable by LISA should be far more frequent than previously assumed,” said LISA lead project scientist Dr. Nora Luetzgendorf, an astronomer at ESA.

“It will most likely make us adjust our models for LISA rates in this mass range. This is just the tip of the iceberg.”

The discovery is reported in a paper in the Monthly Notices of the Royal Astronomical Society.

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Hannah Übler et al. 2024. GA-NIFS: JWST discovers an offset AGN 740 million years after the Big Bang. MNRAS 531 (1): 355-365; doi: 10.1093/mnras/stae943