An Einstein Cross typically forms when the gravity of foreground galaxies bends light from a background galaxy into four distinct images. The fifth central image surprised researchers, as it could not be explained by visible matter alone.
"This discovery gives us a rare chance to study that invisible structure in detail," said Andrew Baker, Distinguished Professor of Physics and Astronomy at Rutgers. "We only know it's there because of how it affects the things we can see, like the way it bends light from distant galaxies."
French astronomer Pierre Cox first noticed the anomaly while analyzing data from the Northern Extended Millimeter Array (NOEMA) in the French Alps. Observations of the distant dusty galaxy HerS-3 with NOEMA and the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile confirmed five distinct images instead of four.
At first, the team suspected a data error. "We thought it was a problem with the instrument. But it was real," Cox recalled. Modeling work by Rutgers astrophysicist Charles Keeton and graduate student Lana Eid demonstrated that only the addition of a dark matter halo could reproduce the observed lensing pattern.
"The only way to make the math and the physics line up was to add a dark matter halo," said Keeton. "That's the power of modeling. It helps reveal what you can't see."
Beyond uncovering hidden matter, the lensing magnifies HerS-3, allowing astronomers to probe its internal structure with unprecedented detail. The team predicts that future observations may reveal features such as outflowing gas, providing further tests of their models.
"This system is like a natural laboratory," Cox said. "We can study both the distant galaxy and the invisible matter that's bending its light."
The work highlights the importance of international collaboration and U.S. scientific infrastructure, including ALMA, the Very Large Array, and NASA's Hubble Space Telescope, all of which supported the study.
Research Report:HerS-3: An Exceptional Einstein Cross Reveals a Massive Dark Matter Halo
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