The European Space Agency's (ESA) Euclid space telescope has captured images of a rare phenomenon known as the Einstein ring. This ring of light demonstrates the extreme curvature of space caused by the galaxy's gravity.
Euclid began its six-year mission to explore the dark Universe on July 1, 2023. However, before the spacecraft could begin full operations, a team of scientists and engineers on Earth had to make sure that all systems were working properly. During this early testing phase, in September 2023, Euclid sent back its first images. Although they were deliberately out of focus, in one of them, mission archivist Bruno Altieri noticed a hint of a very special phenomenon.
"I look at the data from Euclid as it comes in," Bruno explains. "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."
Einstein ring, an extremely rare phenomenon, has been found hidden in plain sight in the relatively close galaxy NGC 6505. This galaxy is located about 590 million light-years from Earth, which is considered a "neighborhood" by cosmic standards. However, thanks to the high-resolution instruments of Euclid, a ring of light around its center has been detected for the first time.
The ring itself is formed by the light from a much brighter galaxy located farther away. This background galaxy is 4.42 billion light-years away, and its light has been bent by gravity on its way to us. The distant galaxy has not been observed before and does not yet have a name.
"“An Einstein ring is an example of strong gravitational lensing," explains Conor O'Riordan from the Max Planck Institute for Astrophysics (Germany), the lead author of the first scientific paper on the analysis of the ring. "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."
Albert Einstein's General Theory of Relativity predicts that light bends around objects in space, and they focus light like a giant lens. This gravitational lensing effect is stronger for more massive objects, such as galaxies and clusters of galaxies. This means that sometimes we can see light from distant galaxies that would otherwise be hidden.
If the alignment is perfect, the light from the distant source galaxy bends to form an impressive ring around the object in the foreground. Such "Einstein rings" are a valuable laboratory for scientists. Studying their gravitational effects can help us learn more about the expansion of the Universe, detect the influence of invisible dark matter and dark energy, and investigate a background source whose light is distorted by dark matter between us and the source.
"“I find it very intriguing that this ring was observed within a well-known galaxy, which was first discovered in 1884," said Valeria Pettorino, Euclid project manager at ESA. "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."
Unlocking the secrets of dark matter and dark energy, which together make up 95% of the Universe, is the main goal of the €1 billion Euclid mission. The telescope, capable of detecting galaxies up to 10 billion light-years away, aims to create the largest three-dimensional map of space. This will allow astronomers to determine the large-scale distribution of dark matter, the mysterious force that accelerates the expansion of the Universe.