A Perfect Ring for Euclid

The galaxy NGC 6505: The Einstein ring created by this gravitational lens can be seen in the centre of the image (Image: ESA/Euclid/Euclid Consortium/NASA, image processing by J.-C. Cuillandre, T. Li)

The Euclid mission of the European Space Agency (ESA) has discovered its first strong gravitational lens: the image of a distant galaxy that appears in the form of a ring, thanks to the gravitational pull of a galaxy much closer to us that happens to be on the same line of sight. The results of the research, led by an international collaboration involving researchers from the University of Bologna, the Italian National Institute of Astrophysics (INAF), the Italian National Institute of Nuclear Physics (INFN) and many Italian universities, were published today in Astronomy & Astrophysics.

Launched in July 2023, Euclid is scanning the deep sky to create the most precise 3D map of the Universe ever made, looking back as far as 10 billion years to study cosmic history and investigate the mysteries of the enigmatic dark matter and dark energy. The mission received a strong Italian contribution through the Italian Space Agency (ASI), INAF, INFN and several academic institutions such as the University of Bologna and must collect a huge amount of data to achieve its ambitious scientific aims. Many surprises are hidden within this data.

One of the first surprises is the galaxy NGC 6505, which has been known since the end of the nineteenth century. It is relatively close to us – its light started its journey “only” 590 million years ago. Thanks to Euclid, it has been discovered that this galaxy acts as a gravitational lens, deflecting light from another far more distant galaxy, whose light started its journey 4.42 billion years ago. The result is an image of the distant galaxy which is distorted enough to form a perfect ring. The research is led by Conor O’Riordan of the Max Planck Institute for Astrophysics in Munich, Germany.

According to Einstein’s theory of general relativity, bodies with mass “bend” the fabric of spacetime that pervades the Universe, deflecting the path of any other object in the vicinity, including light. This phenomenon, called gravitational lensing, produces distorted images of celestial bodies, just like those created by a common magnifying glass. The Euclid mission will use gravitational lensing in its “weak” form to study the invisible dark matter through its influence on the slightly deformed images of billions of galaxies. In rare cases, such as when galaxies at different distances from us are fortuitously aligned, gravitational lensing manifests itself in its most striking form, called “strong gravitational lensing”, producing multiple images of the same galaxy or occasionally an entire ring, known as Einstein ring.

Einstein ring details, a distorted image of a distant galaxy created by the gravitational lens NGC 6505 (Image: ESA/Euclid/Euclid Consortium/NASA, image processing by J.-C. Cuillandre, T. Li)

"This first strong gravitational lens discovered by Euclid has unique characteristics”, explains Massimo Meneghetti, researcher at the National Institute of Astrophysics, associated with the National Institute of Nuclear Physics, one of the authors of the new study. “It is rare to find a galaxy relatively close to us, such as this one found in the NGC (New General Catalogue, a catalogue of nearby galaxies), which acts as a strong gravitational lens. Nearby galaxies are generally not able to focus the light from background sources and produce multiple images, unless they contain huge amounts of matter in their central regions. The formation of complete Einstein rings, like that of NGC 6505, is an even rarer event because it requires the lens galaxy and the source galaxy to be perfectly aligned with our telescope. For these reasons, we do not expect Euclid to observe many lenses such as NGC 6505. Even considering the large area of sky that will be covered during the mission, we expect to discover at most 20 lenses like this”.

This gravitational lens was discovered by chance, in one of the first areas of the sky observed by Euclid: Astronomer Bruno Altieri from ESA identified it while analysing verification phase data of the mission just two months after the launch. For this reason, the research group nicknamed it “Altieri lens”. Although the galaxy NGC 6505 was first observed in 1884, the Einstein ring discovered by Euclid had never been noticed before, demonstrating the mission’s extraordinary capabilities to make new discoveries.

The distortion induced by gravitational lensing depends on the distribution and density of matter of the galaxy acting as a lens. For this reason, by analysing the distortion, it is possible to measure its mass both in terms of stars and dark matter. Additionally, since the Einstein ring of the Altieri lens has a smaller radius than that of NGC 6505, it has been possible, in this case, to accurately study the composition and structure of the central regions, where dark matter is less prominent and the stars are more prevalent.

“Since gravitational lensing is the most precise method to measure mass, combining the model of the Einstein ring and the distribution of stars of the galaxy, we were able to determine that the mass fraction composed of dark matter at the centre of the lens is only 11 percent”, explains co-author Giulia Despali, researcher at the Department of Physics and Astronomy “Augusto Righi” of the University of Bologna, associated with INAF and INFN. “Let us remember that dark matter constitutes about 85 percent of the total matter of our Universe, so the central regions of galaxies are quite distinct. We have measured the properties of the galaxy with extreme precision, discovering a complex structure that varies with the distance from the centre and estimating the initial mass function, that is, the proportion of low-mass and high-mass stars. Therefore, the new Euclid observations help us understand more about both the dark universe and the formation and evolution processes of galaxies”.

Although this discovery happened by chance, within the Euclid collaboration, a large group is dedicated to searching for gravitational lenses. Over one hundred thousand are expected to be found in the 14 thousand square degrees of sky that will be observed during the mission. These investigations make use of both state-of-the-art tools such as artificial intelligence and citizen science, involving the non-expert public in the visual inspection of images, in collaboration with the Zooniverse platform. The aim is to create a detailed map of matter distribution -both visible and dark -in galaxies and galaxy clusters at various distances from the local Universe to study the nature and evolution of dark matter and dark energy over time.