A close-up view of the center of the NGC 6505 galaxy, with the bright Einstein ring around its nucleus, captured by ESA’s Euclid space telescope. The Einstein ring is formed by gravitational lensing, with the mass of galaxy NGC 6505 bending and magnifying the light from a more distant galaxy into a ring. NGC 6505 is a well-known galaxy only around 590 million light-years from Earth, and Euclid’s discovery of a spectacular Einstein ring here was unexpected. (Credit: ESA/Euclid/Euclid Consortium/NASA, image processing by J.-C. Cuillandre, T. Li)
MUNICH — When the European Space Agency’s Euclid space telescope launched on July 1, 2023, to explore the dark universe, no one expected its first major discovery to come during routine testing. Yet in September 2023, while examining deliberately unfocused test images meant to check for ice contamination, international astronomers spotted something extraordinary.
“Even from that first observation, I could see it, but after Euclid made more observations of the area, we could see a perfect Einstein ring. For me, with a lifelong interest in gravitational lensing, that was amazing,” explains Euclid Archive scientist Bruno Altieri, in a statement.
What Altieri had discovered, as reported in the astronomy journal Astronomy & Astrophysics, was a pristine ring of light encircling a galaxy named NGC 6505, located approximately 590 million light-years from Earth—practically our cosmic backyard. This phenomenon, known as an Einstein ring, occurs when light from a distant galaxy gets bent and warped by the gravitational pull of another galaxy sitting directly between it and Earth, creating a cosmic magnifying glass effect.
Most Einstein rings are found around galaxies billions of light-years away, making them difficult to study in detail. NGC 6505’s relative proximity allows scientists to examine this phenomenon with unprecedented clarity. The ring itself is remarkably complete and bright. Most gravitational lenses appear as partial arcs or distorted shapes rather than perfect circles.
The background galaxy whose light forms the ring sits much deeper in space, about 4.42 billion light-years away. Its light has traveled across the cosmos only to be caught and bent by NGC 6505’s gravitational field, creating this perfect circular illusion. Remarkably, this background galaxy had never been observed before and remains unnamed.
“An Einstein ring is an example of strong gravitational lensing,” says lead author Conor O’Riordan of the Max Planck Institute for Astrophysics. “All strong lenses are special, because they’re so rare, and they’re incredibly useful scientifically. This one is particularly special because it’s so close to Earth, and the alignment makes it very beautiful.”
What makes this discovery even more remarkable is that NGC 6505 isn’t a newly discovered galaxy. Astronomers have known about it since 1884.
“The galaxy has been known to astronomers for a very long time. And yet this ring was never observed before,” says ESA Euclid Project Scientist Valeria Pettorino. “This demonstrates how powerful Euclid is, finding new things even in places we thought we knew well.”
This is more than just a pretty cosmic picture. By studying how the background galaxy’s light gets distorted, scientists can measure both the visible and invisible mass in NGC 6505. This analysis revealed something unexpected. Only about 11% of the mass creating the ring comes from dark matter, far less than typically seen in galaxies. This unusually low dark matter fraction challenges current theories about how galaxies like NGC 6505 form and evolve.
To understand why this discovery matters, it helps to know what Euclid was designed to study. The telescope’s main mission focuses on solving one of astronomy’s biggest mysteries: the nature of dark matter and dark energy. Dark matter is an invisible form of matter that can only be detected through its gravitational effects, while dark energy is the unknown force causing our universe to expand at an accelerating rate.
Using its state-of-the-art cameras and instruments, Euclid can detect both strong gravitational lensing, like the Einstein ring around NGC 6505, and weak gravitational lensing, subtle distortions in the shapes of distant galaxies caused by dark matter’s gravitational pull. The telescope combines a visible-light camera offering sharp, detailed images with a near-infrared instrument that helps measure distances to far-off galaxies.
The chances of finding such a pristine Einstein ring around a nearby galaxy are remarkably slim. Computer models suggest that a galaxy like NGC 6505 has only a one in 2,000 chance of producing such a strong gravitational lensing effect. Until now, scientists have discovered fewer than 1,000 strong gravitational lenses, and even fewer have been imaged in such high resolution.
To study this cosmic ring in detail, researchers combined Euclid’s observations with data from ground-based telescopes, including the powerful Keck Cosmic Web Imager in Hawaii. This allowed them to measure the speeds at which stars move within NGC 6505 and determine how mass is distributed throughout the galaxy. The team also measured the galaxy’s total mass within the Einstein ring at approximately 153 billion times the mass of our Sun.
The discovery of this Einstein ring also demonstrates the incredible precision of Euclid’s instruments. The telescope can detect subtle variations in the ring’s shape that reveal how mass is distributed within NGC 6505. These measurements show that the galaxy’s mass isn’t perfectly symmetric but has a slightly boxy shape, providing clues about how it formed and evolved over billions of years.
While this perfectly formed ring represents Euclid’s first major discovery, it’s just the beginning of the telescope’s scientific journey. On February 14, 2024, Euclid began its detailed survey of the sky, embarking on an ambitious mission to create the most extensive 3D map of the universe ever made. Over its planned six-year mission, the telescope will map more than a third of the sky, observing billions of galaxies out to distances of 10 billion light-years.
Scientists expect Euclid to discover around 100,000 new strong gravitational lenses during its mission, though few are likely to match the pristine geometry of Altieri’s lens. Each of these cosmic magnifying glasses will help astronomers study galaxies too distant to observe directly and map the distribution of dark matter across cosmic time.
Technological advances continue to reveal new wonders in well-studied parts of the cosmos. Despite NGC 6505 being known to astronomers for nearly 140 years, its Einstein ring remained hidden until Euclid’s sensitive instruments could reveal it. This suggests that many more astronomical treasures await discovery, even in places we think we know well.
For now, this perfectly formed Einstein ring stands as both a testament to Euclid’s capabilities and a preview of discoveries to come. The telescope’s combination of wide-field coverage and precise measurements promises to transform our understanding of the universe’s dark side, one observation at a time.
“Euclid is going to revolutionize the field, with all this data we’ve never had before,” says O’Riordan.
The fact that this Einstein ring went unnoticed in a well-studied galaxy for over a century raises an intriguing question: how many other astronomical phenomena are hiding in plain sight, waiting for the right technology to reveal them? As Euclid continues its mission to map the dark universe, the answer may prove surprising.
Paper Summary
Methodology
The research team combined observations from multiple instruments to study this Einstein ring. Euclid’s visible-light and near-infrared cameras provided high-resolution images showing the ring’s structure. Ground-based spectroscopy from the Keck Cosmic Web Imager measured the motion of stars within NGC 6505, while the Dark Energy Spectroscopic Instrument provided additional data about the galaxy’s composition. Researchers used sophisticated computer modeling to analyze how the background galaxy’s light gets distorted, revealing the distribution of mass in NGC 6505.
Results
The team measured NGC 6505’s total mass within the Einstein ring as approximately 153 billion solar masses, with only 11% coming from dark matter – an unusually low fraction. They found the galaxy’s central region contains stars formed through a different process than typically observed. The Einstein ring itself measures about 2.5 arcseconds in radius, and the background galaxy creating it sits at a redshift of 0.406 (about 4.42 billion light-years away).
Limitations
The researchers note that their mass measurements depend on certain assumptions about the galaxy’s dark matter distribution. While Euclid’s images provide exceptional detail, even higher resolution observations could reveal finer structures in the Einstein ring. The team also acknowledges that their models of the galaxy’s mass distribution may be simplified compared to reality.
Discussion and Takeaways
This discovery demonstrates Euclid’s exceptional capabilities and provides a unique opportunity to study galaxy formation and dark matter distribution. The unusually low dark matter fraction in NGC 6505’s center challenges current models of galaxy evolution. The finding also suggests that many more gravitational lenses await discovery, even in relatively nearby galaxies.
Funding and Disclosures
The research involved numerous space agencies and institutions across Europe, including the European Space Agency, national space agencies, and research councils. The study utilized data from both Euclid and ground-based telescopes, including Keck Observatory. The researchers declare no competing interests.
Publication Information
The study, “Euclid: A complete Einstein ring in NGC 6505?” was published in Astronomy & Astrophysics (Volume 694, Article A145) in 2025. The research was conducted by an international team led by C. M. O’Riordan, representing one of Euclid’s first major discoveries.
