Covering a huge area of the sky in three mosaics, the data release also includes numerous galaxy clusters, active galactic nuclei and 500 gravitational lens candidates – cases where light from distant galaxies has been bent and distorted by normal and dark matter in the foreground.

The release also includes many transient phenomena – astronomical events that are temporary or short-lived relative to cosmic history. These include supernovae (explosions of stars at the end of their life cycles), gamma-ray bursts (extremely energetic explosions observed in distant galaxies), and fast radio bursts (brief but intense bursts of radio waves from unknown sources in space).

Professor Carole Mundell, ESA’s Director of Science, said: “Euclid shows itself once again to be the ultimate discovery machine. It is surveying galaxies on the grandest scale, enabling us to explore our cosmic history and the invisible forces shaping our Universe.

“With the release of the first data from Euclid’s survey, we are unlocking a treasure trove of information for scientists to dive into and tackle some of the most intriguing questions in modern science. With this, ESA is delivering on its commitment to enable scientific progress for generations to come.”

The data is achieved with Euclid’s two instruments: VIS, a high-resolution visible light camera, and NISP, a near-infrared instrument.

Professor Mark Cropper (Mullard Space Science Laboratory at UCL), who led on designing and developing Euclid’s VIS optical camera over 16 years, working with teams at UCL, Open University and across Europe, said: “Euclid is allowing us to understand the universe on another level entirely. It gives us fine detail over a vast scale.

“To pick one example, Euclid found 70,000 globular clusters – very old, tightly packed groups of stars – in the Perseus Cluster of galaxies. And it has found 500 strong gravitational lenses, where light from distant galaxies has been bent by intervening matter – that doubles the number we knew about previously. All this and much more in just a few days of data.”

Euclid has scouted out the the three areas of the sky where it will eventually provide the deepest observations of its mission. In just one week of observations, with one scan of each region so far, Euclid already spotted 26 million galaxies.

The farthest of those are up to 10.5 billion light-years away. The fields also contain a small population of bright quasars that can be seen much farther away.

In the coming years, Euclid will pass over these three regions tens of times, capturing many more faraway galaxies, making these fields truly ‘deep’ by the end of the nominal mission in 2030. 

But the first glimpse of 63 square degrees of the sky, the equivalent area of more than 300 times the full Moon, already gives an impressive preview of the scale of Euclid’s grand cosmic atlas when the mission is complete. This atlas will cover one-third of the entire sky – 14,000 square degrees – in this high-quality detail.

Dr Valeria Pettorino, ESA’s Euclid project scientist, said: “It’s impressive how one observation of the deep field areas has already given us a wealth of data that can be used for a variety of purposes in astronomy: from galaxy shapes, to strong lenses, clusters, and star formation, among others.

“We will observe each deep field between 30 and 52 times over Euclid’s six-year mission, each time improving the resolution of how we see those areas, and the number of objects we manage to observe. Just think of the discoveries that await us.”

To answer the mysteries it is designed for, Euclid measures the huge variety of shapes and the distribution of billions of galaxies very precisely with its VIS camera, while its near-infrared instrument NISP is essential for unravelling galaxy distances and masses.

The new images already showcase this capability for hundreds of thousands of galaxies, and start to hint at the large-scale organisation of these galaxies in the cosmic web. These filaments of ordinary matter and dark matter weave through the cosmos, and from these, galaxies formed and evolved.

This is an essential piece in the puzzle towards understanding the mysterious nature of dark matter and dark energy, which together appear to make up 95% of the Universe.

Professor Ofer Lahav (UCL Physics & Astronomy) said: “We are thrilled to see Euclid’s stunning galaxy images, and the mapping of their positions and properties within the cosmic web – offering key perspectives into the ‘nature vs nurture’ debate on galaxy evolution.

“This achievement marks the culmination of two decades of dedicated efforts by over 2,000 scientists in both instrumentation and science, including significant contributions from UCL researchers across two departments (Mullard Space Science Laboratory and Physics & Astronomy).

“Some of the most exciting discoveries from Euclid are still ahead, including groundbreaking insights into the mysterious nature of dark energy and dark matter.”

Euclid is expected to capture images of more than 1.5 billion galaxies over six years, sending back around 100 GB of data every day. Such an impressively large dataset creates incredible discovery opportunities, but huge challenges when it comes to searching for, analysing and cataloguing galaxies. The advancement of artificial intelligence (AI) algorithms, in combination with thousands of human citizen science volunteers and experts, is playing a critical role.

Dr Mike Walmsley, Euclid Consortium scientist based at the University of Toronto, Canada, who has been heavily involved in astronomical deep learning algorithms for the last decade, said: “We’re at a pivotal moment in terms of how we tackle large-scale surveys in astronomy. AI is a fundamental and necessary part of our process in order to fully exploit Euclid’s vast dataset.

“We’re building the tools as well as providing the measurements. In this way we can deliver cutting-edge science in a matter of weeks, compared with the years-long process of analysing big surveys like these in the past.”

A major milestone in this effort is the first detailed catalogue of more than 380,000 galaxies, which have been classified according to features such as spiral arms, central bars, and tidal tails that infer merging galaxies. The catalogue is created by the ‘Zoobot’ AI algorithm. During an intensive one-month campaign on Galaxy Zoo last year, 9,976 human volunteers worked together to teach Zoobot to recognise galaxy features by classifying Euclid images. 

When the distortions are very apparent, it is known as ‘strong lensing’, which can result in features such as Einstein rings, arcs, and multiple imaged lenses.

Using an initial sweep by AI models, followed by citizen science inspection, expert vetting and modelling, a first catalogue of 500 galaxy-galaxy strong lens candidates is released today, almost all of which were previously unknown. This type of lensing happens when a foreground galaxy and its halo of dark matter act as a lens, distorting the image of a background galaxy along the line of sight towards Euclid. 

With the help of these models, Euclid will capture some 7,000 candidates in the major cosmology data release planned for the end of 2026, and in the order of 100,000 galaxy-galaxy strong lenses by the end of the mission, around 100 times more than currently known.

Euclid will also be able to measure ‘weak’ lensing, when the distortions of background sources are much smaller. Such subtle distortions can only be detected by analysing large numbers of galaxies in a statistical way. In the coming years, Euclid will measure the distorted shapes of billions of galaxies over 10 billion years of cosmic history, thus providing a 3D view of the distribution of dark matter in our Universe.

• Embedded: This image shows examples of gravitational lenses that Euclid captured in its first observations of the Deep Field areas. Credit: ESA/Euclid/Euclid Consortium/NASA, image processing by M. Walmsley, M. Huertas-Company, J.-C. Cuillandre

Mark Greaves

E: m.greaves [at] ucl.ac.uk

+44 (0)20 3108 9485