Euclid makes dark matter visible

One of the most fascinating effects of Einstein's theory of general relativity is the deflection of light rays by massive objects. When light from distant galaxies passes by a massive object—such as a galaxy cluster—on its way to Earth, it is deflected. This causes background galaxies to appear slightly distorted and stretched from Earth.

Tim Schrabback from the Institute for Astro- and Particle Physics, together with an international team, has now precisely measured this weak gravitational lensing effect for the first time using Euclid data. The target object was the massive galaxy cluster Abell 2390, located approximately 2.4 billion light-years from Earth and observed as part of the Euclid Early Release Observations program.

Shape measurement of thousands of galaxies

Since the undistorted appearance of galaxies is unknown, distortion cannot be deduced from observing a single galaxy. Instead, the research team analyzed the shapes of thousands of galaxies simultaneously. Gravitational distortions manifest as weak but measurable deformations of the galaxies tangential to the center of mass. To ensure the robustness of their results, Schrabback's team employed three independent shape-measuring algorithms, whose results showed good agreement.

To select the relevant background galaxies, the scientists combined the Euclid images with complementary observations from the Subaru Telescope in Hawaii. Using photometric redshifts, they were able to ensure that primarily objects behind the galaxy cluster under investigation were included in the analysis.

Dark Matter Map

From the measured distortion, the team reconstructed the mass distribution within Abell 2390. The vast majority of this mass—and thus the gravitational force that distorts the light from distant galaxies—comes from dark matter. This dark matter neither absorbs nor emits electromagnetic radiation and therefore remains invisible to conventional telescopes. Launched in 2023, the Euclid space telescope makes dark matter indirectly visible by measuring its gravitational effect on the light from distant galaxies.

"Our study has shown that this galaxy cluster is about 1.5 quadrillion times more massive than our Sun. This result is consistent with previous studies of the galaxy cluster but is significantly more precise thanks to the deep and sharp Euclid images," summarizes Tim Schrabback.

Euclid's unique potential

What makes Euclid so special is the combination of exceptional sharpness and a vast field of view. Because the telescope operates outside Earth's atmosphere, it remains unaffected by its image-distorting effects. Its field of view is 180 times larger than that of the Hubble Space Telescope. The observations from Abell 2390 analyzed as part of the pilot study cover less than 0.004 percent of the sky area that Euclid is designed to survey during its entire mission.

"There is enormous scientific potential in the coming years," says Tim Schrabback. The results of this initial demonstration study pave the way for the large-scale cosmological analyses with which Euclid, starting in 2027, will track the growth of cosmic structures throughout Earth's history, thus providing new insights into the nature of dark matter and dark energy.

The observations from Abell 2390 analyzed as part of the pilot study cover less than 0.004 percent of the sky area that Euclid is designed to survey during its entire mission. In addition to Laila Linke and Sebastian Grandis, two junior researchers, Henning Jansen and Florian Kleinebreil, from the Innsbruck research group led by Tim Schrabback, played a key role in the study. The research was funded, among other sources, through the Austrian space program via the Austrian Research Promotion Agency (FFG) and the Austrian Science Fund (FWF). The research group led by Francine Marleau at the University of Innsbruck is also significantly involved in the Euclid mission.