University of California Irvine: UCI, UC Riverside astronomers discover galactic protocluster in early universe

An international team of astronomers, including researchers from the University of California, Irvine and UC Riverside, has discovered a massive cluster of young galaxies forming in the early universe. The growing cosmic metropolis, named MAGAZ3NE J095924+022537, is a massive newborn galaxy cluster, or protocluster, consisting of at least 38 members and is about 11.8 billion light-years away from Earth.

Galaxy clusters get larger over time under gravity and, in the present-day universe, can contain hundreds or even thousands of galaxies, as well as hot gas and dark matter. Eventually, their galaxies burn through the fuel available and evolve from prolific star-producing galaxies into red and dead galaxies.

“In the early universe, all protoclusters discovered until now are full of vigorously star-forming galaxies,” said Ian McConachie, a graduate student in the UC Riverside Department of Physics & Astronomy and lead author of a research paper on the detection published in The Astrophysical Journal. “But incredibly, unlike all of the other protoclusters which have been found at this epoch, many galaxies in MAGAZ3NE J0959 appear to have already stopped forming stars.”

Co-author Mike Cooper, UCI professor of physics & astronomy, said that J0959 was discovered as part of an ongoing survey designed to find and study the early universe’s most massive galaxies.

“These observations offer a unique opportunity to study how the most massive structures in our universe formed,” he said. “We are seeing this protocluster as it appeared when the universe was less than 2 billion years old.”

Co-author Benjamin Forrest, a postdoctoral scholar formerly at UC Riverside now at UC Davis, explained that at the heart of MAGAZ3NE J0959 is a galaxy containing a mass of more than 200 billion suns.

“Why this ultramassive galaxy and so many of its neighbors formed most of their stars and then became inactive when the universe was still so young, in contrast to other known protoclusters from the same time, is a big mystery,” he said.

Forrest noted that MAGAZ3NE J0959 was discovered from the ground. The advent of powerful new capabilities, such as the recently launched James Webb Space Telescope, should soon reveal whether there are other protoclusters like MAGAZ3NE J0959 packed with dead galaxies waiting to be found in the early universe. Should such protoclusters be detected in large numbers, the current paradigm of protocluster formation would require a major revision. A new concept of protoclusters existing in a diversity of states in the early universe would have to be adopted. With many member galaxies quenching – a process by which they lose cold gas and produce fewer stars – in the first 2 billion years, a new model of protocluster evolution would almost certainly pose significant challenges for current models of galaxy formation.

The team used observations from the W.M. Keck Observatory’s Multi-Object Spectrograph for Infrared Exploration to make detailed measurements of MAGAZ3NE J0959 and precisely quantify its distance.

Closely associated with the question of how ultramassive galaxies form is the question of the environment in which they form, e.g., are they always found in extremely dense regions like protoclusters, or can they also form in isolation? Next, the team plans a comprehensive study of the other ultramassive galaxies in the MAGAZ3NE survey to try to answer this question.

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