Figure 1: Dark matter in the Perseus Cluster. The distribution of dark matter (in blue) is overlayed on an image taken by Hyper Sprime-Cam on the Subaru Telescope. The newly detected subcluster located near the galaxy NGC 1264 lies about 1.4 million light-years to the west (right side of the image) of Perseus’s central galaxy, NGC 1275. A faint bridge connects the two structures. An original image without text can be found here (1 MB). (Credit: HyeongHan et al.)
An international team of astronomers has solved one of the longstanding
cosmic mysteries by uncovering direct evidence of a massive, long-lost
object that collided with the Perseus cluster. Using high-resolution
data from the Subaru Telescope, the researchers successfully traced the
remnant of this ancient merger through the dark matter distribution.
Galaxy clusters, composed of thousands of galaxies bound together by
gravity, are among the most massive structures in the Universe. They
grow through energetic mergers — some of the most powerful events since
the Big Bang.
Located about 240 million light-years from Earth,
the Perseus cluster has a mass equivalent to 600 trillion Suns (called
solar masses). For decades, astronomers believed it had long since
settled into a stable, post-merger state. Its apparent lack of clear
merger signatures earned it the reputation of being the "textbook
example" of a relaxed cluster. However, advances in observational
techniques have allowed researchers to peer deeper into its structure,
uncovering subtle yet compelling evidence of past disruption. This
raised a fundamental mystery: if there are signs of a collision, where
is the object that collided with it?
To solve the mystery, the
team analyzed archival data from Hyper Suprime-Cam on the Subaru
Telescope. Gravitational lensing—a phenomenon where massive objects bend
the light from background galaxies—served as a powerful tool to map the
invisible dark matter. Through this technique, the researchers
identified a massive clump of dark matter, weighing approximately 200
trillion solar masses, located about 1.4 million light-years west of the
cluster core (
Figure 1). Remarkably, this structure is connected to the
core of the Perseus cluster by a faint but statistically significant
"dark matter bridge," providing direct evidence of past gravitational
interaction between them.
Numerical simulations conducted by the
team suggest that this dark matter substructure collided with the
Perseus cluster roughly five billion years ago. The remnants of that
collision still shape the present-day structure of the cluster.
"This is the missing piece we’ve been looking for," says Dr. James Jee,
corresponding author of the study. "All the odd shapes and swirling gas
observed in the Perseus cluster now make sense within the context of a
major merger."
"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," continues Dr. HyeongHan Kim, the study’s first author.
"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," says Dr. Jee.
These results appeared as HyeongHan et al. "
Direct Evidence of a Major Merger in the Perseus Cluster" in Nature Astronomy on April 16, 2025.
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About the Subaru Telescope
The Subaru Telescope is a large optical-infrared telescope operated by the National Astronomical Observatory of Japan, National Institutes of Natural Sciences with the support of the MEXT Project to Promote Large Scientific Frontiers. We are honored and grateful for the opportunity of observing the Universe from Maunakea, which has cultural, historical, and natural significance in Hawai`i.
