Roughly 700 million light-years from Earth, two galaxies in the Abell 3667 cluster are mid-collision—and one is slowly bleeding stars into the other.
Astronomers have now captured the clearest view yet of what’s left behind: a thin, million-light-year-long bridge of intracluster light. This dim structure is made of stray stars—stars that used to belong to one galaxy but were pulled loose by the intense gravitational forces of the merger.
“This is the first time a feature of this scale and size has been found in a local galaxy cluster,” said Anthony Englert, a Ph.D. candidate at Brown University and lead author of the new study, published August 5 in The Astrophysical Journal.
Galaxies Caught in the Act of Stealing Stars From Each Other
The team was able to image the faint light bridge by stacking 28 hours of observations collected over several years using the Dark Energy Camera in Chile. “It was just a happy coincidence that so many people had imaged Abell 3667 over the years,” Englert said in a statement.
At the top of the bridge sits IC 4965, a disc-shaped galaxy. At the bottom is JO171, a so-called “jellyfish galaxy,” with long tails of gas trailing behind it. As JO171 falls deeper into the cluster, it’s being stripped of gas and stars, leaving behind a glowing trail.
It might look like a faint glow, but it’s a guide. Because intracluster light tends to follow the path of dark matter—the invisible material that makes up most of the universe’s mass—it gives researchers a way to indirectly map what we can’t see. “The distribution of this light should mirror the distribution of dark matter,” said study co-author Ian Dell’Antonio.
It also tells a story. The stars in the bridge didn’t form there; they were pulled out of orbit and stranded in space. For astronomers, that makes the bridge a record of what happened and when, a kind of gravitational crime scene stretched across a million light-years.
And this is likely just the beginning. The upcoming Vera C. Rubin Observatory is expected to make findings like this far more common once it comes online in the next year or two. “What we did is just a small sliver of what Rubin is going to be able to do,” Englert said.
Galaxies crash into each other all the time. What’s rare is catching one in the act of stealing.
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