Data from the Esa Euclid telescope enable precise analysis of an Einstein ring around the galaxy core of NGC 6505 and thus the surrounding dark matter
Scientists using the ESA Euclid space telescope have discovered a rare "Einstein ring" in a galaxy about 590 million light-years from Earth: the gravitationally distorted image of a background galaxy about 4.42 billion light-years away. Using a computer code developed at the Max Planck Institute for Astrophysics, the team calculated the nature of the foreground galaxy, whose gravity curves the light from the background galaxy like a lens. In particular, the model allows them to calculate how the dark matter must be distributed in the gravitational lensing galaxy. The quality of the data allowed an analysis at a previously unattained level.
A ring-shaped arc of light can be seen around the center of the galaxy NGC 6505, which the ESA Euclid telescope captured. NGC 6505 acts as a gravitational lens and bends the light of a distant galaxy. The almost perfect alignment of NGC 6505 and the background galaxy creates a perfect Einstein ring.
© ESA/Euclid/Euclid Consortium/NASA, image processing by J.-C. Cuillandre, G. Anselmi, T. Li
Euclid blasted off on its six-year mission to explore the dark Universe on 1 July 2023. Before the spacecraft could begin its sky 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," explains 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."
A lens telescope as big as a galaxy
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.
© ESA/Euclid/Euclid Consortium/NASA, image processing by J.-C. Cuillandre, G. Anselmi, T. Li
The Einstein Ring, an extremely rare phenomenon, turned out to be hiding in plain sight in a galaxy not far away. The galaxy, called NGC 6505, is around 590 million light-years from Earth, a stone's throw away in cosmic terms. But this is the first time that the ring of light surrounding its centre is detected, thanks to Euclid's high-resolution instruments. 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. "An Einstein ring is an example of strong gravitational lensing," explains Conor O'Riordan, of the Max Planck Institute for Astrophysics (MPA), Germany, and lead author of the first scientific paper analysing 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."
Top-quality data
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.
"With the repeated Euclid observations of the same field, the data for this Einstein ring is so good that it was a challenge to model the system accurately", O'Riordan points out. The team used the state-of-the-art gravitational lensing code 'pronto' developed at MPA to model the light of the ring at an unprecedented level. "We even had to look at some of the raw data to better understand how the detector works."
CHasing Dark Matter
Modelling the Einstein ring was, however, only the first step. "These types of objects are incredibly useful to study the dark matter substructures in the lensing galaxy, which we'll explore in a subsequent publication," adds O'Riordan. "Euclid is going to revolutionise the field, with all this data we've never had before."
Although this Einstein ring is stunning, Euclid's main job is searching for the more subtle effects of weak gravitational lensing, where background galaxies appear only mildly stretched or displaced. To detect this effect, scientists will need to analyse billions of galaxies. 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. The space telescope will map more than a third of the sky, observing billions of galaxies out to 10 billion light-years. It is expected to find around 100 000 strong lenses, but to find one that's so spectacular - and so close to home - is astonishing. Until now, less than 1000 strong lenses were known, and even fewer were imaged at high resolution. Such an amazing find, so early in its mission, means Euclid is on course to uncover many more hidden secrets.
