Using the European Space Agency’s Euclid space telescope, astronomers have accidentally found a complete Einstein ring around a nearby galaxy known as NGC 6505. This optical effect shows a distorted image of a far-away galaxy as a ring around a separate galaxy in the foreground. NGC 6505’s proximity to Earth and the completeness of its ring make this a rare find.
“An Einstein ring is an example of strong gravitational lensing,” explained study lead Conor O’Riordan of the Max Planck Institute for Astrophysics in Germany. “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.”
Gravitational lensing occurs when massive objects such as black holes, galaxies, or galaxy clusters bend the light emitted by objects behind them. Gravity affects the path light takes through space, as predicted by Albert Einstein’s theory of general relativity. The immense gravity of massive objects bends the light more and thus causes a stronger lensing effect.
Suppose the foreground object — or lens — is positioned directly between the background object and the observer. In that case, the lens distorts the background object’s image so much that it forms a ring around the foreground object. NGC 6505’s alignment with its background object is so precise that the image forms a complete Einstein ring.
Graphical explanation of how a gravitational lens forms an Einstein ring. (Credit: ESA)
Astronomers are interested in gravitational lenses, as they can help study invisible dark matter. When astronomers measure the masses of galaxies, they find a number much higher than can be explained by matter visible in observations. The missing mass is therefore known as dark matter, but its nature remains one of the greatest unanswered questions in astronomy.
As both regular matter and dark matter distort light, gravitational lenses provide an excellent opportunity to study the nature of dark matter. Strong lenses, like NGC 6505, allow astronomers to study dark matter in the lens itself.
Euclid observed NGC 6505 a few months after its launch on July 1, 2023, when teams were still testing the telescope. As part of these commissioning procedures, Euclid took images that were intentionally out of focus in September 2023. The galaxy was in one of these blurry images, which already showed hints of its striking features.
“I look at the data from Euclid as it comes in,” explains Euclid Archive Scientist Bruno Altieri. “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.”
Euclid’s view of NGC 6505 (center). This image combines four sets of observations using Euclid’s Visible Camera (VIS) for brightness and the lower resolution Near-Infrared Spectrometer and
Photometer (NISP) for color data. (Credit: ESA/Euclid/Euclid Consortium/NASA, image processing by J.-C. Cuillandre, G. Anselmi, T. Li)
Soon after, astronomers set out to collect observations of NGC 6505, which O’Riordan’s team proposes to name Altieri’s lens, after the ring’s discoverer. The team studied four observations by Euclid and used ground-based observations to confirm Euclid’s findings. Additionally, the Keck Cosmic Web Imager (KCWI) in Hawaii and the Dark Energy Spectroscopic Instrument (DESI) in Arizona provided insight into the spectrum of light emitted by the galaxy and its ring.
What makes the discovery remarkable is how close the lens is to Earth. The galaxy was first discovered by Lewis A. Swift on June 27, 1884, and is located roughly 590 million light-years from Earth. Only about five lenses have been found this close to home.
“I find it very intriguing that this ring was observed within a well-known galaxy, which was first discovered in 1884,” said ESA Euclid project scientist Valeria Pettorino. “The galaxy has been known to astronomers for a very long time. And yet this ring was never observed before. This demonstrates how powerful Euclid is, finding new things even in places we thought we knew well. This discovery is very encouraging for the future of the Euclid mission and demonstrates its fantastic capabilities.”
The distorted image that forms the Einstein ring is of a previously unknown galaxy located 4.42 billion light-years from Earth. The team modeled the lens’s properties to reconstruct an undistorted view of what the background galaxy may look like.
(Left) Euclid VIS data of the lensed images. (Right) reconstructed image of the background galaxy, using the same units as the data. (Credit: O’Riordan et al.)
Euclid is on a six-year mission to map nearly a third of the sky, during which scientists expect the telescope to discover over 100,000 strong lenses. Altieri’s lens is the first and will remain only one of the few discovered this close to Earth as the team expects Euclid to find no more than 20.
For Euclid’s core science mission, astronomers are interested in optical distortions caused by weak lensing. By statistically analyzing the slight distortions weak lensing imparts on 1.5 billion galaxies, scientists want to map the distribution of dark matter throughout the universe.
While unraveling dark matter is the telescope’s primary purpose, Euclid will deliver a broad range of scientific findings. The early discovery of Altieri’s lens shows the telescope’s potential for studying strong lenses. “Euclid is going to revolutionize the field, with all this data we’ve never had before,” said O’Riordan.
O’Riordan et al.’s results were published in the journal Astronomy & Astrophysics on Feb. 10.
(Lead image: Euclid’s view on NGC 6505 and its Einstein ring. Credit: ESA/Euclid/Euclid Consortium/NASA, image processing by J.-C. Cuillandre, G. Anselmi, T. Li)
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