Euclid telescope maps 900 million galaxies in first major data release

Release contains 15,000 strong lensing candidates that could test dark matter theories

The European Space Agency’s Euclid space telescope has released its first major data set, offering researchers an unprecedented view of the cosmos: more than 900 million galaxies and about 15,000 probable cases of strong gravitational lensing, a magnification effect previously considered rare that scientists said could help unlock the secrets of dark matter.

Andy Taylor, a professor at the University of Edinburgh who leads the UK science ground segment for the mission, called the release the biggest of its kind. “It’s the biggest high-resolution image of the universe we’ve ever had,” Taylor said.

Covering about 14% of the total area of sky that will be mapped by Euclid, the newly released data captures more than 900 million galaxies as well as more than 60 million potential quasars — blazing objects at the centres of distant galaxies, powered by supermassive black holes. Among the images published with the release was Abell 3128, a cluster of hundreds of galaxies lying about 800 million light-years from Earth.

Euclid launched in July 2023 with the promise of creating the largest cosmic map ever made, encompassing a third of the sky and more than 10 billion years of cosmic history. Its goal is to shed light on dark energy and dark matter, thought to make up about 70% and 25% of the universe respectively. Taylor said that while bigger surveys exist, none has the detail and information content of Euclid; by contrast, the Hubble telescope has produced images of incredible resolution but does not cover as large an area of the sky. “The technological leap that’s Euclid is that we can keep that high resolution, but now we can basically map huge volumes of the sky, huge volumes of the universe,” Taylor said.

Strong gravitational lensing occurs when one galaxy, often embedded in a galaxy cluster, sits in front of another, more distant galaxy. According to Taylor, Einstein predicted that the gravitational influence of the foreground galaxy would distort space-time, bending the light from the background galaxy and creating the appearance of bright arcs or rings. The effect is significantly boosted by the presence of dark matter, which increases a galaxy’s mass and hence its gravitational influence.

Taylor said that by studying strong gravitational lensing, scientists may be able to rule out different theories on the nature of dark matter. That is because the lensing is influenced by how the mysterious substance is distributed in a galaxy — a distribution that itself depends on its properties. Stephen Serjeant, a professor of astronomy at the Open University, agreed. “With strong gravitational lensing, you find out how the dark matter is clumping and that’s very diagnostic of the type of matter that it is, because different types of matter can clump in very different ways,” Serjeant said.

“So if you’re looking for where the next breakthrough will be in the properties of dark matter and what dark matter actually is, there’s a good chance – I can’t say it’s certain – but there’s a good chance it might come from strong gravitational lensing,” Serjeant said.

The release expands the catalogue of known strong lenses dramatically. Taylor noted that while experts previously knew of about 1,000 cases of strong gravitational lensing, the Euclid data contains about 15,000 probable cases. An estimated 170,000 are expected to be discovered by the end of the mission, Taylor said.

A further data release is expected to land next summer, Taylor said, which will include results on both the distribution of galaxies and weak gravitational lensing — subtle distortions in distant objects caused by dark matter in the foreground. “We can use that pattern of distortions to actually make a map of where all the dark matter is in the universe,” Taylor said. Scientists could then work out its statistical properties and compare the results with different theories. “Depending on the type of dark matter, the type of dark energy, we’re expecting to see slightly different patterns on the sky,” Taylor said.

With Euclid generating ever-more data, Taylor said there is plenty left to explore. “It’s opening up that high resolution of the whole sky, the whole universe. So it just means there are plenty of objects that we’ll never have seen before, plenty of research phenomena,” Taylor said. “No human eyeball is able to look at all of the Euclid data any more. It’s now too big.”