An international team of astronomers has observed a rare configuration knows as an “Einstein Cross”, where the light from a very distant galaxy is bent by the gravitation of a group of nearer galaxies. The result is five images of the same distant galaxy, in a cross formation. In a new study, the researchers show that the image configuration can only be explained by the presence of massive dark matter associated with the nearer galaxies. This discovery provides a unique laboratory to probe dark matter and to better understand the evolution of galaxies in the early Universe.
Strong gravitational lensing occurs when light from a distant galaxy passes by a massive galaxy (or a group of galaxies) that distorts spacetime. This causes the path of light of the background source to bend, making it appear brighter and producing multiple images, arcs, or a ring. The resulting amplification enables one to study galaxies in the early universe in great detail.
The new study: “HerS-3: An Exceptional Einstein Cross Reveals a Massive Dark Matter Halo” is published in the Astrophysical Journal by lead author Pierre Cox. Chalmers astronomer Kirsty May Butler, co-author to the study, recalls seeing the Einstein Cross for the first time while working at IRAM, Institute Radio Astronomie Millimétrique in France.
“I was sitting at my desk at IRAM, with colleagues Pierre Cox and Roberto Neri standing behind me when I made the first image of HerS-3 from the observational data. When the image loaded on the screen, I remember saying, 'Wow, is that real?!’ to them. It turns out it was, very much, real,” says Kirsty.
A large unseen component
The light from HerS-3 is bent by four massive galaxies in the foreground. But, when the astronomers tried to reproduce the arrangement of the five images of the Einstein cross, it was only possible when adding a large, unseen component: a concentration of dark matter associated with the group of galaxies. Only by adding this massive component does the source reconstruction precisely match the properties of the five images.
“In the beginning, we all assumed this would be a relatively straightforward job of modelling the bright lensing galaxies seen lying in front of HerS-3. When co-author Charles Keeton's modelling started to suggest a much more massive structure, invisible in all our observations, there was definitely a feeling that we were onto something – quite literally – big”, says Kirsty May Butler.
Current theories suggest that dark matter, which accounts for about 80 percent of the mass of the Universe, is made of as-yet-unidentified particles that don’t interact with visible light. However, due to its large mass, dark matter can be identified by its gravitational influence. The estimated mass of the dark matter in the case of HerS-3 amounts to a few trillion solar masses.
The image and study were made possible using observations from the Northern Extended Millimeter Array (NOEMA, Institut de Radioastronomie Millimétrique – IRAM, France), the Atacama Large Millimeter/submillimeter Array (ALMA, Chile), the Karl G. Jansky Very Large Array (VLA, USA), and the Hubble Space Telescope (HST, NASA/ESA).
“This is a nice example of how, in astronomy, it is often necessary to combine observations from many different telescopes and the expertise of many different people before things start to make sense”, says Kirsty.
More info:
Read more about the discovery in the press release from IRAM: An Exceptional Einstein Cross Reveals Hidden Dark Matter.
The scientific paper: HerS-3: An Exceptional Einstein Cross Reveals a Massive Dark Matter Halo, by Pierre Cox, et al was published in September 2025 in the Astrophysical Journal. Chalmers astronomers Kirsty May Butler and Tom JLC Bakx are authors and part of the research team.
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- Postdoc, Astronomy and Plasma Physics, Space, Earth and Environment
- Postdoc, Astronomy and Plasma Physics, Space, Earth and Environment