An international team of scientists, including experts from the University of Portsmouth, has made a rare discovery using the Euclid space telescope - an Einstein ring, a circle of light formed around a galaxy by gravitational lensing.
The Einstein Ring turned out to be hiding in plain sight in a galaxy not far away. The galaxy, called NGC 6505, is a stone's throw away in cosmic terms. But this is the first time that the ring of light surrounding its centre has been detected, thanks to Euclid's high-resolution instruments.
Professor Thomas Collett , from the University of Portsmouth's Institute of Cosmology and Gravitation , said: "This lens is particularly special because it's in our cosmic backyard: a 'mere' 500 million light years away.
"In fact, it's so close that astronomers first observed the galaxy in the 1880s, but only with the resolving power of Euclid can we now reveal that it's a gravitational lens. Nearby lenses are exciting because they allow us to test the validity of General Relativity on astronomical scales."
The ring of light surrounding the centre of the galaxy NGC 6505, captured by ESA's Euclid telescope, is a stunning example of an Einstein ring.
Euclid blasted off on its six-year mission to explore the dark Universe on 1 July 2023. Before the spacecraft could begin its survey, the team of scientists and engineers on Earth had to make sure everything was working properly.
During this early testing phase, in September 2023, Euclid sent some images back to Earth. They were deliberately out of focus, but in one fuzzy image Euclid Archive Scientist Bruno Altieri saw a hint of a very special phenomenon and decided to take a closer look.
"I look at the data from Euclid as it comes in," explained Bruno. "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."
The ring around the foreground galaxy is made up of light from a farther out bright galaxy. This background galaxy is 4.42 billion light-years away, and its light has been distorted by gravity on its way to us. The far-away galaxy hasn't been observed before and doesn't yet have a name.
Conor O'Riordan, of the Max Planck Institute for Astrophysics, Germany, is lead author of the first scientific paper analysing the ring. He said: "An Einstein ring is an example of strong gravitational lensing . 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."
This lens is particularly special because it's in our cosmic backyard: a 'mere' 500 million light years away. In fact, it's so close that astronomers first observed the galaxy in the 1880s, but only with the resolving power of Euclid can we now reveal that it's a gravitational lens.
Professor Thomas Collett, Professor of Astrophysics
Albert Einstein's general theory of relativity predicts that light will bend around objects in space, so that they focus the light like a giant lens. This gravitational lensing effect is bigger for more massive objects - galaxies and clusters of galaxies. It means we can sometimes see the light from distant galaxies that would otherwise be hidden.
If the alignment is just right, the light from the distant source galaxy bends to form a spectacular ring around the foreground object. These Einstein rings are a rich laboratory for scientists.
Studying their gravitational effects can help us learn about the expansion of the Universe, detect the effects of invisible dark matter and dark energy , and investigate the background source whose light is bent by dark matter in between us and the source.
A close-up view of the centre of the NGC 6505 galaxy, with the bright Einstein ring around its nucleus, captured by ESA's Euclid space telescope.
Professor Collett added: "This is the first of 100,000 strong gravitational lenses that we're expecting to find with Euclid, but we don't expect to find any closer to Earth than this. Most will be 10 times more distant.
"The proximity of NGC6505 and the Einstein ring in the centre has allowed us to study the stars and dark matter distribution in NGC6505. We've found that there is more mass in stars than expected, which is indicative of an excess of faint low mass stars compared to the stellar population in the Milky Way."
By exploring how the Universe has expanded and formed over its cosmic history, Euclid will reveal more about the role of gravity and the nature of dark energy and dark matter. The space telescope will map more than a third of the sky, observing billions of galaxies out to 10 billion light-years.
Euclid began its detailed survey of the sky on 14 February 2024 and is gradually creating the most extensive 3D map of the Universe yet. Such an amazing find, so early in its mission, means Euclid is on course to uncover many more hidden secrets.
