An international team of astronomers using the NASA/ESA/CSA James Webb Space Telescope has discovered evidence of an ongoing merger between two galaxies and their massive black holes from when the Universe was just 740 million years old. This represents the most distant detection of a black hole merger ever recorded and marks the first time this phenomenon has been observed so early in the Universe’s history.
Supermassive black holes, with masses ranging from millions to billions of times that of the Sun, are found in most massive galaxies in the local Universe, including our own Milky Way. These black holes likely play a significant role in the evolution of their host galaxies. However, the mechanisms by which these black holes grew to such immense sizes remain poorly understood. The discovery of enormous black holes already present in the first billion years after the Big Bang suggests that their growth must have occurred very rapidly and early.
The James Webb Space Telescope is now providing new insights into the growth of black holes in the early Universe. The latest Webb observations have revealed evidence of a merger between two galaxies and their massive black holes when the Universe was only 740 million years old. This system is identified as ZS7. Massive black holes that actively accrete matter exhibit distinctive spectrographic features that astronomers can use to identify them. These signatures, especially for very distant galaxies like those in this study, are inaccessible from ground-based telescopes and can only be detected with Webb.
“We found evidence for very dense gas with fast motions near the black hole, as well as hot and highly ionized gas illuminated by the energetic radiation typically produced by black holes during their accretion episodes,” explained lead author Hannah Übler of the University of Cambridge, UK. “Thanks to the unprecedented sharpness of its imaging capabilities, Webb also allowed our team to spatially separate the two black holes.”
The team discovered that one of the black holes has a mass 50 million times that 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,” added team member Roberto Maiolino of the University of Cambridge and University College London, UK. “Our findings suggest that merging is an important route through which black holes can rapidly grow, even at cosmic dawn,” Übler continued. “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 the two black holes will generate gravitational waves once they merge. Such events will be detectable with the next generation of gravitational wave observatories, like the upcoming Laser Interferometer Space Antenna (LISA) mission, recently approved by the European Space Agency. LISA will be the first space-based observatory dedicated to studying gravitational waves. “Webb’s results are telling us that lighter systems detectable by LISA should be far more frequent than previously assumed,” shared LISA Lead Project Scientist Nora Luetzgendorf of the European Space Agency in the Netherlands. “It will most likely make us adjust our models for LISA rates in this mass range. This is just the tip of the iceberg.”
This discovery was made as part of the Galaxy Assembly with NIRSpec Integral Field Spectroscopy program. The team has recently been awarded a new Large Programme in Webb’s Cycle 3 of observations to study the relationship between massive black holes and their host galaxies in the first billion years in detail. An important component of this program will be to systematically search for and characterize black hole mergers. This effort aims to determine the rate at which black hole mergers occur in early cosmic epochs and to assess the role of merging in the early growth of black holes and the production of gravitational waves from the dawn of time.