Astronomers spot a rare 'Einstein Cross' with a hidden twist

Astronomers spotted an “Einstein Cross” with a twist: a faint fifth image glowing at the center. That extra speck shouldn’t exist – unless something unseen is warping space more than expected.

Careful modeling showed the culprit was a massive halo of dark matter wrapped around the lensing galaxies. The system also magnifies a far-off galaxy, turning the universe into its own telescope.

An international team, including Rutgers University researchers, traced the strange pattern and built the case for the invisible mass.

The analysis, led by Pierre Cox with Rutgers astrophysicists Charles Keeton and Andrew Baker, and graduate student Lana Eid, was published in The Astrophysical Journal.

The discovery began as a head-scratcher shared between colleagues and grew into a detailed investigation of how unseen matter sculpts light across billions of light-years.

Einstein Cross that breaks the rules

It all started with a surprising image. “Have you ever seen an Einstein Cross with an image in the middle?” Baker asked. The center is normally dark in these rare alignments.

“I said, ‘Well, that’s not supposed to happen,’” said Keeton. “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 forms when the gravity of foreground galaxies bends and splits the light from a single, much more distant galaxy into four distinct images.

Nature draws a symmetric cross from one background source. A fifth image at the center breaks the usual playbook. It hints that the mass distribution in front isn’t just the visible stars and gas; something larger and smoother must be adding extra curvature to space.

Clues from radio telescope data

The first clue came from radio telescope data taken in France. “We were like, ‘What the heck?’” said Cox, who noticed the anomaly in observations from the Northern Extended Millimeter Array (NOEMA) in the French Alps.

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

The target was a dusty, distant galaxy called HerS-3. Follow-up with Chile’s Atacama Large Millimeter/submillimeter Array (ALMA) confirmed it: five images, not four.

At first the team suspected a glitch. “We tried to get rid of it,” Cox said. “We thought it was a problem with the instrument. But it was real.”

Einstein crosses and dark matter

With the data in hand, Keeton and Eid built lens models to explain the geometry and brightness of each image. The four visible foreground galaxies weren’t enough.

“We tried every reasonable configuration using just the visible galaxies, and none of them worked,” said Keeton.

“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.”

Dark matter cannot be observed directly, but its gravity leaves fingerprints. “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,” Baker said. “This discovery gives us a rare chance to study that invisible structure in detail.”

A natural laboratory

The odd layout isn’t just pretty. It’s useful. The lensing boosts and stretches HerS-3, letting astronomers resolve features they’d otherwise miss.

At the same time, the cross and the central dot pin down how the lensing group distributes visible and invisible mass.

“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.”

For Eid, the project was a front-row seat to discovery across continents. “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.”

“Collaborating across continents and time zones taught me the value of diverse expertise and research styles in fully understanding a new discovery.”

Hunting for new physics

Good models make risky bets. The team expects to see additional features in future observations, such as outflowing gas from the background galaxy.

“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.”

Either outcome teaches something: confirmation validates the mass map; a mismatch sends the team hunting for new physics or overlooked structure.

The universe bends its rules

The research highlights why gravitational lensing is such a powerful probe of dark matter. It lets astronomers infer the shape and heft of invisible halos that wrap galaxies and groups.

It also showcases global infrastructure that makes these insights possible.

“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,” Baker said. “We hope they will continue to enable such discoveries well into the future.”

A tiny, impossible-looking dot in the middle of a cosmic cross turned out to be a beacon from the dark. To make it appear, the universe needed more gravity than starlight accounts for – a sweeping halo of dark matter.

With clever modeling and far-flung telescopes, researchers turned a curiosity into a map of the invisible, and a single system into a classroom for studying both a distant galaxy and the hidden scaffolding of the cosmos.

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