A new discovery captures the cosmic moment when a galaxy cluster — among the largest structures in the universe — started to assemble only about a billion years after the big bang, one or two billion years earlier than previously thought possible. This result, made using NASA’s Chandra X-ray Observatory and James Webb Space Telescope, will lead astronomers to rethink when and how the largest structures in the universe formed.

The findings are described in a paper published today in the journal Nature.

The object, known as JADES-ID1 for its location in the “JWST Advanced Deep Extragalactic Survey,” (JADES) has a mass about 20 trillion times that of the Sun. Astronomers classify JADES-ID1 as a “protocluster” because it is currently undergoing an early, violent phase of formation and will one day turn into a galaxy cluster. However, JADES-ID1 is found at a much larger distance — corresponding to a much earlier time in the universe — than astronomers expected for such systems, providing a new mystery of how it could form so quickly.

“This may be the most distant confirmed protocluster ever seen,” said Akos Bogdan of the Center for Astrophysics | Harvard & Smithsonian (CfA) who led the study. “JADES-ID1 is giving us new evidence that the universe was in a huge hurry to grow up.”

Galaxy clusters contain hundreds or even thousands of individual galaxies immersed in enormous pools of superheated gas, along with large amounts of unseen dark matter. Astronomers use galaxy clusters to measure the expansion of the universe and the roles of dark energy and dark matter, among other important cosmic studies.

“It’s very important to actually see when and how galaxy clusters grow,” said co-author Gerrit Schellenberger, also of CfA. “It’s like watching an assembly line make a car, rather than just trying to figure out how a car works by looking at the finished product.”

The Chandra and Webb data reveal that JADES-ID1 contains the two properties that confirm the presence of a protocluster: a large number of galaxies held together by gravity (Webb sees at least 66 potential members) that are also sitting in a huge cloud of hot gas (detected by Chandra). As a galaxy cluster forms, gas falls inward and is heated by shock waves, reaching temperatures of millions of degrees and glowing in X-rays.

What makes JADES-ID1 exceptional is the remarkably early time when it appears in cosmic history. Most models of the universe predict that there likely would not be enough time and a large enough density of galaxies for a protocluster of this size to form only a billion years after the big bang. The previous record holder for a protocluster with X-ray emission is seen much later, about three billion years after the big bang.

“We thought we’d find a protocluster like this two or three billion years after the big bang — not just one billion,” said co-author Qiong Li from the University of Manchester in the UK. “Before, astronomers found surprisingly large galaxies and black holes not long after the big bang, and now we’re finding that clusters of galaxies can also grow rapidly.”

After billions of years JADES-ID1 should evolve from a protocluster into a massive galaxy cluster like those we see much closer to Earth.

To find JADES-ID1, astronomers combined deep observations from both Chandra and Webb. By design, the JADES field overlaps with the Chandra Deep Field South, the site of the deepest X-ray observation ever conducted. This field is thus one of the few in the entire sky where a discovery such as this could be made. In an earlier study, a team of researchers led by Li and Conselice found five other proto-cluster candidates in the JADES field, but only in JADES-ID1 are the galaxies embedded in hot gas. Only JADES-ID1 possesses enough mass for an X-ray signal from hot gas to be expected.

“Discoveries like this are made when two powerful telescopes like Chandra and Webb stare at the same patch of sky at the limit of their observing capabilities,” said co-author Christopher Conselice, also from the University of Manchester. “A challenge for us now is to understand how this protocluster was able to form so quickly.”

The earlier study led by Li was published in the Monthly Notices of the Royal Astronomical Society.

NASA's Marshall Space Flight Center in Huntsville, Alabama, manages the Chandra program. The Smithsonian Astrophysical Observatory's Chandra X-ray Center controls science operations from Cambridge, Massachusetts, and flight operations from Burlington, Massachusetts.