Euclid opens data treasure trove to study the Dark Universe

Today, 19 March 2025, the European Space Agency’s Euclid mission released its first batch of survey data, including a preview of its deep fields. Here, hundreds of thousands of galaxies in different shapes and sizes take centre stage and show a glimpse of their large-scale organisation in the cosmic web. These have been observed and analysed by scientists of the Euclid Consortium, demonstrating the unprecedented power of this telescope designed to provide the most precise map of our Universe over time. The scientific results are described in a series of 27 scientific publications together with 7 technical papers that describe how this data has been processed that have been published today in arXiv. 

Covering a huge area of the sky in three mosaics, the data release also includes numerous galaxy clusters, active galactic nuclei (AGN) and transient phenomena, as well as the first classification survey of more than 380,000 galaxies and 500 gravitational lens candidates compiled through combined artificial intelligence and citizen science efforts. All of this sets the scene for the broad range of topics that Euclid is set to address with its rich dataset.

“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,” says ESA’s Director of Science Carole Mundell. “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 we now make public represent a small fraction of the total data Euclid will collect”, comments Francisco Javier Castander, researcher from the Institute of Space Studies of Catalonia (IEEC — Institut d’Estudis Espacials de Catalunya) at the Institute of Space Sciences (ICE-CSIC). “Nevertheless, even such a small fraction of the data has allowed us to perform many scientifically relevant studies that we now present.”

The data unveiled today provide a first glimpse of Euclid’s cosmological survey. The so-called Q1 fields are illustrative of what will be extensively analysed by scientists within the Euclid Collaboration to map the large-scale structure of the Universe across cosmic time, and investigate the nature of dark matter and dark energy in the years to come. With a sky area of about 63 square degrees, the equivalent area of more than 300 times the full Moon, these observations represent the largest contiguous areas of sky ever observed with an optical/near-infrared space telescope. 

“The sky coverage achieved so far is unprecedented, and the images continue to showcase the outstanding performance of the satellite’s instruments. I am truly excited to publish the cosmological results in the coming years,” comments Cristobal Padilla, Institute of High Energy Physics (IFAE) researcher.

“The images captured by the Euclid telescope that are now available to the international scientific community, once processed, are impressively deep and detailed, allowing astronomers to familiarise themselves with the data the mission will generate and plan its scientific exploitation beyond the cosmological objectives set by the consortium,” explains Rafael Toledo, researcher at the Polytechnic University of Cartagena (UPCT).

Thanks to Euclid’s very wide field of view and high resolution, these exquisite data are also highly valuable for various astrophysical studies on smaller scales, ranging from clusters of galaxies to planet-sized objects. All the papers published today are dedicated to this non-cosmological science, also called legacy science.

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.

In August 2024, members of the Euclid Consortium launched a first citizen-science campaign on the Zooniverse platform, enlisting thousands of volunteers to train a deep-learning algorithm classifying galaxy morphologies. The resulting catalogue, based on the first 0.45% of the ∼100 million lower-redshift galaxies that Euclid will ultimately capture in detail, has already proven valuable to researchers. Moreover, thanks to this large volume of high-quality data, scientists were able to observe differences with respect to simulated galaxy shapes and features. They also investigated how different environments, star-formation rates, and morphologies are linked and drive the evolution of galaxies in various epochs of our Universe.

In that respect, the study led by Marc Huertas-Company, staff researcher from the Instituto de Astrofísica de Canarias (IAC), used Euclid’s unique combination of wide field of view and resolution together with the models trained thanks to volunteers from all over the world participating in the citizen science campaign to quantify the abundance of bars and how they change with cosmic time.

“The Q1 data release demonstrates the transformative power of the Euclid telescope for the study of galaxy physics. It also confirms the growing impact of AI for the analysis of increasingly voluminous and complex data in astrophysics. Approximately 50% of the scientific articles published with these first Euclid data are based on AI methods,” adds Marc Huertas-Company. “The IAC has led the development of deep neural networks for the study of galaxy morphology thanks to a citizen science campaign. In just a few months of observations, Euclid and AI have identified 10 times more barred galaxies than in more than twenty years of observations with the Hubble Space Telescope and the James Webb Space Telescope.”

By means of another citizen-science campaign with the help of over 1,000 participants and newly trained machine-learning algorithms, more than 500 strong gravitational lens candidates at galaxy scale were identified in the Q1 fields. “The resolution of the Euclid data allowed us to see these lensed systems in incredible detail, and these data are only a small fraction of the lenses we hope to find in the coming years as the survey continues,” says Philip Holloway, PhD student at the University of Oxford. These rare phenomena, predicted by Einstein’s general relativity, are invaluable tools for understanding the distribution of dark matter around galaxies, studying internal dynamics in galaxy clusters, and even uncovering previously hidden galaxies. So far, only about 150 of these lenses had been observed by space telescopes.

Euclid maps the Universe across cosmic time and traces the evolution of large-scale structures that form the so-called cosmic web. Using the Q1 fields, researchers have successfully recovered galaxy filaments and explored their role in galaxy morphology and alignment far beyond previous limits, as well as which properties and environments make galaxy clusters connect to these filaments.

“For the first time, we are studying the cosmic web in a statistical way on a very large area.” says Simona Mei, Professor at Université Paris Cité (CNRS/IN2P3/APC). As key building blocks of the cosmic web, numerous previously unknown galaxy clusters have been discovered and characterised – some from the first ages of our Universe, many exhibiting strong gravitational lensing features.

The next data release from the Euclid Consortium will concern Euclid’s nominal survey and core-science, including results about the nature of dark energy. A first worldwide data release is currently planned for October 2026. At least two other quick releases and two other data releases are expected before 2031, the currently foreseen end date of Euclid’s main survey.

About the IEEC

The Institute of Space Studies of Catalonia (IEEC — Institut d’Estudis Espacials de Catalunya) promotes and coordinates space research and technology development in Catalonia for the benefit of society. IEEC fosters collaborations both locally and worldwide and is an efficient agent of knowledge, innovation and technology transfer. As a result of more than 25 years of high-quality research, done in collaboration with major international organisations, IEEC ranks among the best international research centres, focusing on areas such as: astrophysics, cosmology, planetary science, and Earth Observation. IEEC’s engineering division develops instrumentation for ground- and space-based projects, and has extensive experience in working with private or public organisations from the aerospace and other innovation sectors.

The IEEC is a non-profit public sector foundation that was established in February 1996. It has a Board of Trustees composed of the Generalitat de Catalunya, Universitat de Barcelona (UB), Universitat Autònoma de Barcelona (UAB), Universitat Politècnica de Catalunya · BarcelonaTech (UPC), and the Spanish Research Council (CSIC). The IEEC is also a CERCA centre.