For over a century, astronomers have searched the skies for a perfect example of a phenomenon predicted by Albert Einstein: a luminous halo formed by the bending of light around a massive object. Now, thanks to the European Space Agency’s $1 billion Euclid telescope, scientists have found one of the most pristine examples yet — a complete Einstein ring in the elliptical galaxy NGC 6505.
This dazzling image, featuring the galaxy NGC 6505, offers astronomers a unique opportunity to probe the mysteries of dark matter, dark energy, and the very fabric of the universe.
A Cosmic Lens in Plain Sight
The Einstein ring, a near-perfect circle of light surrounding NGC 6505, is the result of a cosmic alignment. A distant galaxy, located 4.42 billion light-years away, sits directly behind NGC 6505, which lies 590 million light-years from Earth. The gravity of the foreground galaxy bends the light from the background galaxy, creating a ring-like structure.
“This is a beautiful, extraordinary, thrilling and lucky find in our first data,” said Prof. Stephen Serjeant, an astronomer at the Open University. “An Einstein ring as perfect as this is extremely rare.”
What makes this discovery even more remarkable is that NGC 6505 has been known to astronomers since 1884. Yet, the Einstein ring remained hidden until now. “This demonstrates how powerful Euclid is, finding new things even in places we thought we knew well,” said Dr. Valeria Pettorino, ESA Euclid project scientist.
The ring was first spotted by Bruno Altieri, an archive scientist at ESA, during early testing of Euclid’s instruments. “Even from that first observation, I could see it,” Altieri recalled. “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 rings are a manifestation of gravitational lensing, a phenomenon predicted by Albert Einstein’s general theory of relativity. Massive objects, like galaxies, warp the fabric of space-time, bending the path of light that passes near them. When the alignment is just right, this bending creates a ring of light.
Yet complete Einstein rings are exceedingly rare. Even more unusual is finding one so close to Earth. The newly discovered system, described in Astronomy & Astrophysics, lies just 600 million light-years away, making it the first strong gravitational lens ever found in an NGC (New General Catalogue) galaxy.
A Laboratory for Dark Matter and Relativity
This particular Einstein ring could be a scientific goldmine, owing to its relatively close proximity and near-perfect roundness. By studying the ring, astronomers can measure the mass of the foreground galaxy, including its dark matter content. In this case, using data from the Dark Energy Spectroscopic Instrument (DESI), they estimate that about 11% of the galaxy’s mass is dark matter — a relatively small fraction compared to the universe as a whole, where dark matter is thought to make up 27% of the total mass-energy content.
The ring’s proximity to Earth also makes it an ideal laboratory for testing Einstein’s theory of general relativity. “Having an Einstein ring so close to Earth will let us test relativity in ways we can’t with other faraway lenses,” said Collett. Astronomers can measure the galaxy’s mass in two ways: by observing how much the light bends and by studying the speed of the stars within the galaxy. If these measurements don’t match, it could hint at new physics beyond Einstein’s theory.
The faint orange glow surrounding the bright ring is the lensing galaxy itself, while the ring is the distorted light from the background galaxy. “This is probably the prettiest lens we will find in the mission,” Collett added.
Things to Come from Euclid
Launched in July 2023, the telescope is on a six-year mission to map more than a third of the sky, observing billions of galaxies up to 10 billion light-years away. Its primary goal is to uncover the secrets of dark matter and dark energy, which together make up 95% of the universe.
While the Einstein ring is a striking example of strong gravitational lensing, Euclid’s main focus is on weak lensing, where the distortions of background galaxies are far subtler. By analyzing billions of galaxies, scientists hope to trace the large-scale distribution of dark matter and measure the influence of dark energy, the mysterious force driving the accelerated expansion of the universe.
As Euclid continues its survey, astronomers expect to find around 100,000 strong lenses. But few will be as spectacular — or as close — as this one.
