A massive dark matter halo may explain the strange 5th point of this 'Einstein Cross'

Astronomers have discovered a rare cosmic alignment that reveals hidden dark matter, offering a new way to study the invisible substance that makes up most of the universe.

Data from the Northern Extended Millimeter Array (NOEMA) in the French Alps revealed an extra image in the center of what is known as an Einstein Cross — a gravitational lensing effect that causes light from a distant object to bend and appear as four distinct images arranged in a cross-like pattern. In recent observations, the light from a distant, dusty galaxy called HerS-3 was split into five rather than four images, suggesting something unusual was bending the light in this unexpected way, according to a statement from Rutgers University.

An Einstein Cross forms when the gravity of galaxies in the foreground bends and splits the light of a more distant galaxy into four distinct images. However, what puzzled astronomers with regard to the newly studied Einstein Cross this time was a curious fifth image sitting at the center of the cross.

At first, they suspected a data glitch, but the anomaly persisted in repeat observations, including data from the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile. The fifth image could not be explained by the visible foreground galaxies alone. Only after adding a massive, invisible halo of dark matter to their computer models could the researchers reproduce what the radio telescope had observed.

"We tried every reasonable configuration using just the visible galaxies, and none of them worked," Charles Keeton, co-author of the study and a professor at Rutgers, said in the statement. "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 seen directly, but its gravitational effects are evident throughout the cosmos. In this case, it not only created the rare lensing pattern but also magnified HerS-3, allowing astronomers to study the distant galaxy in greater detail and the effects of dark matter.

"This system is like a natural laboratory," Pierre Cox, lead author of the study and research director at the French National Centre for Scientific Research, said in the statement. "We can study both the distant galaxy and the invisible matter that’s bending its light."

The team's models suggest future observations could reveal additional features, such as gas flowing out of the galaxy, which would provide further evidence that dark matter is magnifying the details of HerS-3.

Their findings were published on Sept. 16 in The Astrophysical Journal.