Galaxy clusters are among the largest structures in the Universe, composed of thousands of galaxies bound together by mutual gravity.
Like galaxies, they grow through mergers that are among the most powerful in the cosmos since the Big Bang.
Despite all that we have learned about the large-scale structure of the Universe, there are still many mysteries waiting to be resolved.
This is certainly true of the Perseus Cluster, which has remained a mystery to astronomers for some time.
For decades, astronomers believed this cluster was the product of past mergers that had long since settled into a stable state.
However, advances in observation techniques have uncovered evidence that this cluster experienced disruption in the past.
Thanks to an international team of astronomers led from South Korea, this mystery has been resolved.
Using the Subaru Telescope at Maunakea, Hawaii, the team discovered a “Dark Matter bridge” connecting Perseus to a subcluster of galaxies.
The research was led by Kim HyeongHan, a Ph.D. Candidate at Yonsei University in South Korea.
He was joined by fellow members of the Yonsai Observable UNiverse Group (YOUNG), the University of California Davis, the Harvard & Smithsonian Center for Astrophysics (CfA), Seoul National University, the Korea Advanced Institute of Science and Technology (KAIST), SNU Astronomy Research Center, and Australian Astronomical Optics at Macquarie University.
Their paper, “Direct Evidence of a Major Merger in the Perseus Cluster,” appeared in Nature Astronomy on April 16th, 2025.
Located about 240 million light-years from Earth in the Perseus constellation, the Perseus Cluster contains thousands of galaxies with a mass equivalent to 600 trillion Suns.
For decades, astronomers considered Perseus a textbook example of a “relaxed cluster” since there were no indications that any mergers had taken place for a long time.
However, more recent observations have produced several lines of evidence contradicting this theory about its dynamical state.
These include asymmetric plasma morphology, filamentary galaxy distribution, and ancient features (like large-scale cold fronts). These suggest that the cluster might have experienced a major merger in the past, which triggered the search for Perseus’ missing companion.
For their study, HyeongHan and his colleagues relied on the Gravitational Lensing technique, where massive objects in the foreground (which alter the curvature of spacetime in their vicinity) amplify light from more distant objects.
While analyzing archival data from the Hyper Suprime-Cam on the Subaru Telescope, they identified a massive clump of Dark Matter about 1.4 million light-years from the cluster’s core. Said Dr. James Jee, a corresponding author of the study:
“This is the missing piece we’ve been looking for. All the odd shapes and swirling gas observed in the Perseus cluster now make sense within the context of a major merger.
This breakthrough was made possible by combining deep imaging data from the Subaru Telescope with advanced gravitational lensing techniques we developed —demonstrating the power of lensing to unveil the hidden dynamics of the Universe’s most massive structures.”
This bridge structure, approximately 200 trillion times the mass of the Sun, connects the core of the Perseus Cluster to the newly-discovered subcluster companion.
The discovery of this bridge also provides the first direct evidence of past gravitational interaction between Perseus and a neighboring grouping of galaxies.
“It took courage to challenge the prevailing consensus, but the simulation results from our collaborators and recent observations from the Euclid and XRISM space telescopes strongly support our findings,” added Dr. HyeongHan.
Written by Matthew Williams/Universe Today.
