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 Instituto de Astrofísica de Canarias (IAC) leads several of the 27 scientific publications that, together with 7 technical articles, are presented today in arXiv.

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. 

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”. 

"The data obtained with the two on-board instruments VIS and NISP more than meet the quality and sensitivity specifications we established more than 10 years ago for this space mission. This is a great success of the collaboration between ESA and the Euclid Consortium," says Rafael Rebolo, CSIC professor at the IAC and member of the Euclid Consortium Board of Directors together with Castander. 

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.

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.

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."

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."

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. "The first Euclid data demonstrate its ability to reveal clusters of forming galaxies in the distant Universe with ease," says Helmut Dannerbauer, IAC researcher and co-author of another of the published papers.

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.

Revealing the low-surface-brightness Universe

Another major challenge for Euclid will be to unveil millions of dwarf galaxies, in particular those with low surface brightness, as well as remnant structures from galaxy mergers, tails, and the intergalactic medium. Another published paper reveals a large number of dwarf galaxies lurking in the Universe: "With only a small part of the Q1 data we have been able to detect and study almost 3000 dwarf galaxies. This corresponds to about 180 galaxies per square degree, so the number of dwarf galaxies that will be discovered during the mission will completely change the paradigm in the field of galaxy evolution! With Euclid, dwarf galaxies will become the undisputed protagonists," emphasises Anna Ferré-Mateu, researcher at the IAC and co-author of the paper.

SUBSTELLAR, the IAC project to exploit the mission data to identify an unprecedented number of substellar-mass objects (brown dwarfs and planetary-mass stars), is also leading two of the publications based on the Euclid Q1 data. This programme is funded by the European Research Council through an Advanced Grant. 

According to Eduardo L. Martín Guerrero de Escalante, IAC research professor and one of the two Independent Legacy Scientists of the Euclid mission responsible for SUBSTELLAR: "The Euclid Q1 data have provided us with deep, wide-field images where we have been able to identify more than 5000 objects whose peculiar colours match what is expected for ultra-cold objects that could have substellar masses and contribute to the dark matter of the Milky Way; several dozen of these objects also have spectra that confirm their substellar nature. Two postdoctoral researchers of the project, Carlos Domíguez Tagle and Marusa Zerjal, are leading the first work on substellar science with the Q1 data."

The IAC has also made an outstanding contribution to another article in which 161 explosive transient events were identified, obtaining measurements of the brightness variations in different phases of their evolution, including before, during and after their detection in ground-based studies. "One of the most relevant findings is the early infrared detection of the supernova SN 2024pvw, captured 15 days before its maximum brightness, which represents a significant advance in the study of these phenomena," says Carlos. M. Gutiérrez, researcher at the IAC and co-author of the article.

"Another significant and impressive result is the detection with very deep measurements of the enigmatic core-collapse supernova SN 2023aew, observed 436 days after the maximum of the light curve," says Frédérick Poidevin, an IAC researcher who also participated in the study. "These data, and in this work we are only talking about one epoch of observations, demonstrate the potential of Euclid for transient astronomy and open up new opportunities to study the evolution of supernovae and their impact on the evolution of the Universe," stresses the astrophysicist.

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.

About Euclid and the Spanish contribution

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.

Spain has an important involvement in the mission, with a prominent role in the Consortium that has led the mission from its origin.  On the one hand, 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, as well as the cosmological simulations of the mission.

On the other hand, the IAC and the Polytechnic University of Cartagena (UPCT) have been in charge of the electronic unit that controls the NISP instrument and its start-up software. In fact, the IAC's technological participation has been awarded the EUCLID Consortium EULER Award for its outstanding contribution to the instrumentation of the mission.

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.

Articles with prominent participation of the IAC:

• Euclid Quick Data Release (Q1): “A first look at the fraction of bars in massive galaxies at z < 1”, Euclid Collaboration: Huertas-Company et al. (2025) 

• Euclid Quick Data Release (Q1): “A first look at a multimodal autoregressive foundation model for exploring galaxy properties”, Euclid Collaboration: Siudek et al. (2025)

• Euclid: Quick Data Release (Q1) – “Photometric studies of known transients”, Duffy et al. (2025)

• Euclid: Quick Data Release (Q1) – “A census of dwarf galaxies across a range of distances and environments”, Marleau et al. (2025)

• Euclid: Quick Data Release (Q1) - “Exploring galaxy morphology across cosmic time through Sérsic fits”, Quilley et al. (2025)

• Euclid: Quick Data Release (Q1) - “Optical and near-infrared identification and classification of point-like X-ray selected sources in Q1”, Roster et al. (2025)

• Euclid: Quick Data Release (Q1) Splash Paper – “Combined Euclid and Spitzer galaxy density catalogues at z>1.3 and detection of significant Euclid passive galaxy overdensities in Spitzer overdense regions”, (2025)