Astronomers Discover Rare Einstein Cross With Fifth Image, Revealing Hidden Dark Matter

Scientists find a rare cosmic pattern that will help them learn more about the invisible matter that holds the universe together

When Rutgers theoretical astrophysicist Charles Keeton first saw an unusual picture shared by his colleague, he was intrigued.

"Have you ever seen an Einstein Cross with an image in the middle?" his colleague Andrew Baker asked, referring to a rarely seen cosmic configuration.

Keeton hadn't. The implications were enormous.

"I said, well, that's not supposed to happen," said Keeton, the Vice Provost for Experiential Learning at Rutgers University-New Brunswick. "You can't get a fifth image in the center unless something unusual is going on with the mass that's bending the light."

An "Einstein Cross" is a rarely seen cosmic configuration, in which the light from a distant galaxy is bent by the gravity of galaxies in front of it, creating four images. But the extra image in this Einstein Cross pointed to "something unusual," which turned out to be a massive, hidden halo of dark matter. The existence of this invisible structure could only be inferred through careful computer modeling and analysis.

The discovery, made by an international team that includes Keeton, Baker and Rutgers graduate student Lana Eid, is now being publishedin The Astrophysical Journal.

Dark matter makes up most of the matter in the universe, but it can't be seen directly. "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," said Baker, a Distinguished Professor in the Department of Physics and Astronomy in the School of Arts and Sciences and a co-author of the study. "This discovery gives us a rare chance to study that invisible structure in detail."

The first step toward that discovery was taken in France.

"We were like, 'What the heck?'" said Pierre Cox, a French astronomer, Research Director at the French National Centre for Scientific Research and the study's lead author, who first spotted the anomaly in data from the Northern Extended Millimeter Array (NOEMA) of radio telescopes in the French Alps.

"It looked like a cross, and there was this image in the center," Cox said. "I knew I had never seen that before."

The team was studying a distant, dusty galaxy called HerS-3. Using NOEMA and the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, they saw that the light from HerS-3 split into five rather than four images. At first, they thought it might be a glitch in the data. But the fifth image wouldn't go away.

"We tried to get rid of it," Cox said. "We thought it was a problem with the instrument. But it was real."

Computer modeling of the gravitational lens by Keeton and Eid showed that the four visible foreground galaxies causing the gravitational bending couldn't explain the details of the five-image pattern. Only with the addition of a large, invisible mass, in this case, a dark matter halo, could the model match the observations.

"We tried every reasonable configuration using just the visible galaxies, and none of them worked," said Keeton, also a professor in the Department of Physics and Astronomy and a co-author of the study. "The only way to make the math and the physics line up was to add a dark matter halo. That's the power of modeling. It helps reveal what you can't see."

The unusual configuration doesn't just look cool: the scientists said it's scientifically valuable. The lensing effect magnifies the background galaxy, allowing astronomers to study its structure in greater detail than usual. It also offers a rare chance to learn about the dark matter that surrounds the foreground galaxies.

"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."

Eid, a Rutgers graduate student pursuing her doctoral degree and a co-author of the study, said her involvement in the research project has been exciting from beginning to end.

"I was thrilled to join this project as a graduate student, especially since it involved a fascinating lensing system that grew more intriguing as our models evolved," Eid said. "Collaborating across continents and time zones taught me the value of diverse expertise and research styles in fully understanding a new discovery."

The team has even predicted that more features, such as outflowing gas from the galaxy, could be visible in future observations. If those predictions are confirmed, it would be a powerful validation of their models. If not, it would still teach them something new.

"This is a falsifiable prediction," Keeton said. "If we look and don't see it, we'll have to go back to the drawing board. That's how science works."

Baker said the discovery was critically enabled by both international collaboration and U.S. federal support for science. "ALMA in Chile and the Very Large Array (VLA) in New Mexico are supported by the National Science Foundation, and the Hubble Space Telescope is supported by NASA; all played vital roles in this work," he said. "We hope they will continue to enable such discoveries well into the future."

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