A serendipitous image taken by the European Space Agency’s Euclid telescope allowed astronomers to spot a stunning new example of an Einstein ring.
This extremely rare phenomenon occurs when a large space object acts like a lens and magnifies light from another object directly behind it. This was first predicted by Einstein’s general theory of relativity, which describes how the gravity of large objects, like galaxies, warp the spacetime fabric and alter the path of light.
If the alignment between telescope, foreground object and background object is just right, the magnified light makes a ring.
An Einstein ring can help astronomers test hypotheses about the expansion of the universe as well as the nature of dark matter and dark energy. However, less than 1,000 such “strong lenses” are known, and this new example is remarkably close to Earth.
Astronomers working with the Euclid space telescope made the discovery during early testing when deliberately out of focus test images caught the attention of ESA scientist Bruno Altieri.
“Even from that first observation, I could see it,” says Altieri. “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.”
The galaxy acting as a gravitational lens is called NCG 6505 and it lies 590 million light-years away from Earth, ‘a cosmic stone’s throw away,’ say the authors. However, the light of the object it is magnifying lies more than 4.4 billion light-years away. This object has never been observed before and does not yet have a name.
“I find it very intriguing that this ring was observed within a well-known galaxy, which was first discovered in 1884,” says Valeria Pettorino, ESA Euclid Project Scientist. “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.”
Euclid launched on 1 July, 2023, beginning a 6-year mission to study dark matter, dark energy and gravity. One of its goals is to create a 3D map of the large-scale structure of the universe by observing billions of galaxies across more than a third of the sky.
During its observations, ESA scientists expect the telescope to detect around 100,000 strong lenses, increasing the known number by two orders of magnitude and providing higher resolution images of these phenomena.
“Euclid is going to revolutionise the field, with all this data we’ve never had before,” says Conor O’Riordan, of the Max Planck Institute for Astrophysics, Germany, and lead author of the paper.
While strong lenses like Einstein rings are visually appealing and scientifically valuable, Euclid will focus on the more common phenomena of weak lenses. Rather than create multiple images of a background object, weak lensing causes slight distortions of the background object. Measuring this effect will enable Euclid scientists to make a 3D map of dark matter in the Universe.
Finding an Einstein ring so early in Euclid’s survey of the sky is bolstering the scientific team. “This discovery is very encouraging for the future of the Euclid mission and demonstrates its fantastic capabilities,” says Pettorino.
The research is published in the journal, Astronomy and Astrophysics.
