Astronomers have captured the first optical evidence of a million-light-year-long bridge of stars being torn from one massive galaxy and pulled into another, a rare galactic interaction occurring in the Abell 3667 galaxy cluster, located about 700 million light-years from Earth. The discovery, published August 5 in The Astrophysical Journal, reveals a dramatic cosmic merger between two galaxy clusters, each with its own brightest cluster galaxy (BCG), locked in a violent gravitational dance.
The Universe’s Most Violent Dance
Abell 3667 did not form all at once. Astronomers believe it is the result of two smaller galaxy clusters merging about a billion years ago, each carrying its own dominant central galaxy, known as the brightest cluster galaxy (BCG). Today, those giants are locked in a rapid, aggressive merger, pulling stars away from each other in the process.
The new images, captured with the Dark Energy Camera (DECam) on the Víctor M. Blanco Telescope in Chile, show the bridge of stars stretching between the lenticular galaxy IC 4965 and the spectacular jellyfish galaxy JO171. As JO171 plunges deeper into the cluster, streams of gas are being stripped from its outer ring, shutting down star formation in parts of the galaxy.
Lead researcher Anthony Englert of Brown University called the discovery “a huge surprise,” explaining that while astronomers knew such a bridge could form, “it hadn’t been documented anywhere before now.”
How The Faint Glow Was Captured
Detecting the dim light of the stellar bridge required more than a decade’s worth of telescope time. Englert and his team stitched together 28 hours of observations collected over years by different astronomers using DECam.
“It was just a happy coincidence that so many people had imaged Abell 3667 over the years, and we were able to stack all of those observations together,” Englert said. The result is one of the deepest optical views of the cluster ever produced, revealing the delicate structure of stars floating freely in intergalactic space.
X-ray and radio observations had already suggested Abell 3667 was undergoing a rapid merger, but this is the first optical evidence to confirm it. The bridge not only connects two galaxies but also offers a window into the violent forces shaping the universe.
A New Way To Map Dark Matter
Intracluster light does more than reveal the history of a merger. Because this faint glow tends to follow the same distribution as dark matter, it offers astronomers an indirect way to map this invisible substance, which is thought to make up about 80 percent of the universe’s mass.
“The distribution of this light should mirror the distribution of dark matter, so it provides an indirect way to ‘see’ the dark matter,” said co-author Ian Dell’Antonio of Brown University. Understanding how dark matter moves and interacts during massive collisions could help scientists piece together how cosmic structures evolve.
The discovery also highlights the kind of breakthroughs expected from the upcoming Vera C. Rubin Observatory. Set to begin operations later this year or in early 2026, the Rubin Observatory will carry out the Legacy Survey of Space and Time (LSST), mapping the southern sky in unprecedented detail over a 10-year period.
“What we did is just a small sliver of what Rubin is going to be able to do,” Englert said. “It’s really going to blow the study of the ICL wide open.” With a telescope twice the size of Blanco and the largest digital camera ever built, Rubin will likely uncover hundreds of similar structures.
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