Euclid Discovers Einstein Ring Around Nearby Galaxy – Sky & Telescope

A spectacular surprise has emerged in a routine scan of the cosmos: a nearly perfect “Einstein ring,” hidden in plain sight around a galaxy that astronomers had known about for more than a century. The discovery, made by the European Space Agency’s (ESA) Euclid mission, is a testament to both cosmic serendipity and the telescope’s sharp gaze.

When examining early test images, Euclid Archive Scientist Bruno Altieri (ESA) noticed something unusual: a distorted, out-of-focus galaxy. He assumed it was just another calibration issue; nevertheless, something about this blurry feature piqued his curiosity.

A few weeks later, when Euclid captured a clearer, in-focus image of the same region, Altieri realized he had stumbled on something extraordinary. The image showed a nearly perfect Einstein ring, a rare gravitational-lensing phenomenon first predicted by the general theory of relativity. The discovery was published in Astronomy & Astrophysics.

Gravitational lensing occurs when the gravity of a massive foreground object, like a galaxy, bends and magnifies the light from a more distant object behind it. This effect often produces arcs or distortions in images, but if the alignment between the foreground and background galaxies is just right, the distant galaxy’s light is bent into a complete ring.

What makes this particular discovery so unique is its location. The foreground galaxy, NGC 6505 in the Draco constellation, is relatively nearby — only 590 million light-years from Earth. Most previously discovered Einstein rings occur around far more distant galaxies, making them harder to study in detail. The background galaxy is likewise unusually close compared to other lensed objects — though its light still took 4.42 billion years to reach Earth.

“The probability of finding a lens depends on multiple factors, including how far away the source is and the relative positions of the lens and source,” says Christopher Kochanek(Ohio State University). “This system is rare because both the lens and the source are unusually close to us.”

Moreover, this is the first time an Einstein ring has been found centered around the nucleus of an NGC-class galaxy, a category of general galaxies cataloged as early as the 19th century. American astronomer Lewis A. Swift discovered NGC 6505 in 1884.

How Did No One Spot It Before?

High-resolution imaging is key to detecting gravitational lenses and, surprisingly, neither the Hubble nor the James Webb space telescopes had observed this particular galaxy. “It’s a boring NGC galaxy,” Altieri explains. “There are 2,500 of them. Why would anyone have studied this one in detail?”

Recent observations with ground-based telescopes, such as Subaru’s Suprime-Cam in Hawaii, hint at the ring’s presence, but only if you know exactly what to look for. “If a telescope had been pointed at it in the last few years, you could have seen it,” Altieri adds. “But no one expected anything unusual to be there.”

Machine-learning algorithms are sometimes used to identify gravitational lenses, but in this case, the discovery required human intervention. “It’s a bit of luck, but sometimes the human eye is good at spotting things,” Altieri says.

What can we learn from this ring?

Einstein rings are powerful tools for understanding the universe. They help refine measurements of cosmic distances and expansion rates, because astronomers can use them to study how light from the background galaxy has been stretched and magnified. In this case, the clear image allows a closer look at the background galaxy, too. “Since the source for this ring is a fairly nearby galaxy, we can study the magnified images in detail, especially the dynamics of its stars,” says Kochanek

By analyzing how the light from the background galaxy is bent, scientists can measure the mass of NGC 6505, including its dark matter content. According to initial findings, only about 11% of the mass within the ring is dark matter.

“That’s not surprising,” notes Kochanek. “In many Einstein rings, the lensing occurs farther out in the galaxy, where dark matter contributes more significantly, usually between 25 to 50% of the mass. But here, the ring is much closer to the galaxy’s center, where normal matter dominates.”

What’s Next?

This Einstein ring is just the beginning. “We’re detecting many more Einstein rings in Euclid’s survey,” Altieri says. “But we expect none quite as spectacular as this one.”

Euclid’s wide field of view and sharp vision enables it to cover vast areas of the sky and detect rare objects missed by earlier telescopes. Its main goal is to map more than a third of the sky, cataloging billions of galaxies out to when the universe was only a few billion years old. The mission should find more than 100,000 strong gravitational lenses similar to this one.

But another part of the mission’s goal is to study weak gravitational lensing, in which dark matter more subtly distorts galaxy shapes. By cataloging weak lensing over wide swaths of sky, Euclid will help map the universe’s structure and improve our understanding of dark matter and energy. Mapping dark matter reveals how gravity shapes cosmic structures, and by comparing this with the universe’s rate of expansion, scientists can better understand how dark energy influences that expansion.

For now, this Einstein Ring discovery serves as an early victory for Euclid’s mission, hinting at the hidden wonders it has yet to uncover.