Euclid opens data treasure trove to study the Dark Universe

• The European Space Agency’s Euclid mission released its first batch of survey data, including a preview of its deep fields.

• Using Artificial Intelligence (AI) algorithms in combination with citizen science campaigns, the Euclid Consortium scientific results include the discovery of strong gravitational lensing systems, the exploration of galaxy clusters and the cosmic web, the characterisation of active galactic nuclei (AGN) and quasars, studies on galaxy evolution and morphology, and the identification of numerous dwarf galaxies and transients.

• Spain has an important role in the Euclid mission, with a prominent role in the Consortium that has led the mission from its origin as well as the development of its instrumentation, data processing and scientific exploitation.

• The IFT Gravity and Cosmology group is a permanent member of the Euclid Consortium since 2011.

pie de foto: 

ESA/Euclid/Euclid Consortium/NASA, image processing by J.-C. Cuillandre, E. Bertin, G. Anselmi.

Figure 1: 

Various huge galaxy clusters as well as intra-cluster light, and gravitational lenses are visible in this image of an area of Euclid’s Deep Field South. The cluster near the centre is called J041110.98-481939.3, and is located almost 6 billion light-years away.

On 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 Sciences (ICE-CSIC) and the Institute of Space Studies of Catalonia (IEEC). “Nevertheless, even such a small fraction of the data has allowed us to perform many scientifically relevant studies that we now present”.

First glimpse of Euclid’s cosmological survey

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.

¨Euclid is performing exceptionally well. 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.

AI and citizen science to unravel the mysteries hidden in Euclid data

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.

pie de foto: 

Credits: ESA/Euclid/Euclid Consortium/NASA, image processing by M. Walmsley, M. Huertas-Company, J.-C. Cuillandre.

Figure 2: 

Galaxies in different shapes, all captured by Euclid during its first observations of the Deep Field areas. As part of the data release, a detailed catalogue of 380,000 galaxies was published, which have been classified according to features such as spiral arms, central bars, and tidal tails that infer merging galaxies.

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.

pie de foto: 

ESA/Euclid/Euclid Consortium/NASA, image processing by M. Walmsley, M. Huertas-Company, J.-C. Cuillandre.

Figure 3: 

Examples of gravitational lenses that Euclid captured in its first observations of the Deep Field areas. Using an initial sweep by artificial intelligence models, followed by citizen science inspection, expert vetting and modelling, a first catalogue of 500 galaxy-galaxy strong lens candidates was created, almost all of which were previously unknown.

Another study published today, led IAC researcher by Malgorzata Siudek, presents AstroPT, a powerful AI model that learns from images and light patterns (SEDs) of galaxies, without needing lots of human-labeled data. Thanks to Euclid ultra-sharp images and deep infrared observations across the sky, the IA model is able to learn from better, more diverse data than ever before.

¨We explored the use of foundational AI models, similar to those used on Chat GPT, for data exploration. These intelligent models are capable of performing a multitude of tasks for which they were not specifically trained and open a new avenue for the use of AI in science¨, says Malgorzata Siudek. ¨Euclid's observations, due to their volume and complexity, are ideal for the development of this new generation of AI.¨

The Euclid space mission has also ushered in a new era in the study of AGNs, which represent the bright phase of supermassive black holes at the center of nearly every galaxy. “These data help us better understand how black holes and their host galaxies grow together throughout cosmic history.” says Berta Margalef-Bentabol, Postdoctoral researcher at the Netherlands Institute for Space Research. Using advanced AI algorithms, researchers have established vast catalogs of AGN and red quasars - extremely luminous AGN - featuring thousands of new candidates with unprecedented positional measurements across a large area of the sky, and are now able to statistically confirm how galaxy mergers drive AGN activity.

Tracing out the cosmic web in Euclid’s deep fields

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.

Thanks to Euclid’s vast field of view and its ability to detect faint and distant objects, researchers have further identified thousands of new dwarf galaxy candidates, as well as thousands of mysterious ‘little red dot’ candidates—objects from the early Universe first observed a few years ago with JWST. Euclid has also allowed for the determination of host galaxies for several previously hostless transient bright sources, and the physical properties of dusty, massive red galaxies from the early Universe, demonstrating again Euclid’s capability to deliver highly valuable data across a wide range of objects and cosmic ages.

Future milestones for the Euclid mission

“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,” says Valeria Pettorino, ESA’s Euclid project scientist. “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.”

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.

More information

The data release of 19 March 2025 is described in multiple scientific papers which have not yet been through the peer-review process, but which will be submitted to the journal Astronomy & Astrophysics. A preprint of the papers is available here from 19 March 12:00 CET. Find more detailed information about the data release here.

Euclid ‘quick’ (Q1) releases, such as the one of 19 March, are of selected areas, intended to demonstrate the data products to be expected in the major data releases that follow, and to allow scientists to sharpen their data analysis tools in preparation. The mission’s first cosmology data will be released to the community in October 2026. Data accumulated over additional, multiple passes of the deep field locations will be included in the 2026 release.

The three deep field previews can now be explored in ESASky from 19 March 12:00 CET onwards:

• Euclid Deep Field South

• Euclid Deep Field Fornax

• Euclid Deep Field North

Euclid is a European mission, built and operated by ESA, with contributions from NASA. The Euclid Consortium – consisting of more than 2000 scientists from 300 institutes in 15 European countries, the USA, Canada and Japan – is responsible for providing the scientific instruments and scientific data analysis. Euclid is a medium-class mission in ESA’s Cosmic Vision Programme.

The visible instrument (VIS) and the Near Infrared Spectrometer and Photometer (NISP) instrument were developed and built by a consortium of scientists and engineers from 15 countries, many from Europe, but also from the USA, Canada and Japan.

Spain has an important involvement in the mission, with a prominent role in the Consortium that has led the mission from its origin. The ICE-CSIC, the IFAE and the IEEC have been involved since 2006 in the initial concepts of the mission and have been responsible for the design, construction, assembly and validation tests of the Filter Wheel Assembly (FWA) of the NISP instrument. Together with PIC they are responsible for the cosmological simulation effort in the Euclid mission. UPCT, in collaboration with IAC, have been responsible for the design, construction and validation of the control electronics of the NISP Instrument.

In addition, around 80 European companies participate in Euclid, of which 9 are Spanish, including Airbus, Alter Technology, Crisa, Deimos Space, GTD, Navair, Sener and Thales Alenia Space Spain. In more than 20 Spanish institutions, there are around 100 scientists preparing the scientific exploitation of the mission to unravel the mysteries of the dark universe.

About Euclid

Euclid is a European mission, built and operated by ESA with contributions from NASA. The Euclid Consortium, comprising over 2000 scientists from 300 institutes in 15 European countries, the USA, Canada, and Japan, provides scientific instruments and data analysis. ESA selected Thales Alenia Space as the prime contractor for the satellite and service module construction, with Airbus Defence and Space developing the payload module, including the telescope. NASA supplied the detectors for the Near-Infrared Spectrometer and Photometer (NISP). Euclid is part of ESA’s Cosmic Vision Programme.

The Euclid Consortium is a collaboration of scientists, engineers, and officials funded by national research agencies, responsible for defining the mission's scientific objectives, providing scientific instruments, and processing data. The IFT Gravity and Cosmology group has been a permanent member of the Euclid Consortium since 2011.