A long time ago, in two galaxies far, far away, two massive black holes merged. This happened when the Universe was only 740 million years old. A team of astronomers used JWST to study this event, the most distant (and earliest) detection of a black hole merger ever.
Such collisions are fairly commonplace in more modern epochs of cosmic history and astronomers know that they lead to ever-more massive black holes in the centers of galaxies. The resulting supermassive black holes can contain millions of billions of solar masses. They affect the evolution of their galaxies in many ways.
Using JWST and HST, astronomers have found behemoth black holes earlier and earlier in cosmic time, within the first billion years of the Universe’s history. That raises the question: how did they get so massive so fast? Black holes accrete matter as they grow, and for the most supermassive ones, their colliding galaxies are part of that matter-harvesting history.
The most recent JWST observations focused on a system called ZS7. It’s a galaxy merger where two very early systems come together, complete with colliding black holes. This is not something astronomers can detect with ground-based telescopes. The merger itself lies quite far away. Plus, the expansion of the Universe stretches its light into the infrared part of the electromagnetic spectrum. That makes it inaccessible from Earth’s surface. However, infrared is detectable with JWST’s Near-infrared Spectrometer (NIRSpec). It can find signatures of mergers in the early Universe, according to astronomer Hannah Übler of the University of Cambridge in the United Kingdom.
Zeroing in on the ZS7 galaxy system and the colliding black holes. Courtesy: The field in which the ZS7 galaxy merger was observed by JWST. Courtesy ESA/Webb, NASA, CSA, J. Dunlop, D. Magee, P. G. Pérez-González, H. Übler, R. Maiolino, et. al“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 Übler, who is lead author on a paper about the discovery. “Thanks to the unprecedented sharpness of its imaging capabilities, Webb also allowed our team to spatially separate the two black holes.”
Those black holes are pretty massive: one contains about 50 million solar masses. The other probably has about the same mass, but it’s hard to tell because it’s embedded in a dense gas region. The stellar masses of the galaxies puts them in about the same stellar-mass population as the nearby Large Magellanic Cloud, according to astronomer Pablo G. Pérez-González of the Centro de Astrobiología (CAB), CSIC/INTA, in Spain. “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 analysis of the JWST observations reinforces the idea that mergers are an important way for black holes to grow. That’s particularly true in the early Universe, according to Ühler. “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.”
Many active galactic nuclei (AGN) in the very early Universe are associated with somewhat massive black holes. These are likely part of a general merger process in early epochs. Astronomers want to know when these mergers began. That would help them pinpoint the growth of the central supermassive black holes. Mergers of that kind are a likely route for the growth of black holes so early in cosmic time.
An artist’s impression of two merging black holes. Image: NASA/CXC/A. Hobart
That’s why astronomers are so anxious to spot them with JWST and future telescopes. They hold the key to understanding the evolution of galaxies and black holes in the infancy of the Universe. Uhler and her team members point this out in their paper, saying: “Our results seem to support a scenario of an imminent massive black hole merger in the early universe, highlighting this as an additional important channel for the early growth of black holes. Together with other recent findings in the literature, this suggests that massive black hole merging in the distant universe is common.”
Of course, these mergers don’t just generate light we can detect with JWST. They also generate very faint gravitational waves. But, there’s hope of detecting those waves with the upcoming Laser Interferometer Space Antenna (LISA). It will be in place in the 2030s and should be able to focus on the types of galaxy and black-hole mergers JWST is detecting today in infrared light.
Webb Detects Most Distant Black Hole Merger to Date
GA-NIFS: JWST Discovers an Offset AGN 740 Million Years After the Big Bang
