Euclid Space Telescope Unveils First Data Release

Comprehensive scientific dataset sets standards and provides insights into the depths of the universe - thanks in part to strong German participation

• First major data release: The European Space Agency has released the first major dataset from the Euclid space telescope, launched in July 2023.
• A record number of galaxies discovered: After only a few years of operation, Euclid has already found 26 million galaxies. Over a period of six years, it is expected to take images of more than 1.5 billion galaxies and send about 100 gigabytes of data to Earth every day.
• High sensitivity and a large field of view: Euclid has found the 26 million galaxies in just 0.1 per cent of the sky. After six years, Euclid is expected to have observed one third of the sky. By way of comparison, SDSS, one of the most comprehensive sky surveys to date, discovered about 15 million galaxies with a ground-based telescope observing an area of 35 per cent of the sky.
• The shape of 380,000 galaxies: In addition to the high number of galaxies, Euclid has also identified the structures of individual galaxies. So far, it has been possible to determine the shape and distance of 380,000 galaxies.
• On the trail of dark matter: In the coming years, Euclid will measure billions of galaxies from the last 10 billion years of cosmic history and map for the first time in 3D how dark matter is distributed in the universe.

The newly published data cover a comparatively large area of the sky, 63 square degrees, with three images, each consisting of individual exposures. By the end of the mission, about a third of the entire sky should have been observed. The current data include numerous galaxy clusters, active galactic nuclei and transient phenomena. This first survey data unlocks a treasure trove of information for scientists to dive into and tackle some of the most intriguing questions in modern science. Euclid enables us to explore our cosmic history and the invisible forces shaping our universe.

Euclid is a space telescope with an exceptionally large field of view. In a single shot, it captures an area 240 times larger than the Hubble Space Telescope. It also delivers outstanding image quality in both the visible and infrared light spectrum. The Hubble telescope was built to delve as deeply as possible into comparatively small sections of the sky. Euclid is somewhat less sensitive and so, with the same exposure time, it sees fewer of the particularly distant and therefore extremely faint galaxies. However, this compromise enables Euclid to find a minimum number of galaxies over as large an area as possible, which theoretical astrophysicists need to test their cosmological models and to make sense of the dark matter and dark energy they contain.

Crucial contributions from Germany

When looking at the image, a glimpse of the large-scale structure of the Universe can be seen. This is the organisation of galaxies along the so-called 'cosmic web'. This web consists of huge clusters of galaxies connected to one another by strands of gas and invisible dark matter.

Euclid's Deep Field South covers 28.1 square degrees in the southern constellation of Horologium, the pendulum clock. This field has not been covered to date by any deep sky survey and so has a huge potential for new, exciting discoveries.'>

Euclid is particularly impressive in the infrared channel, for which the Max Planck Institute for Extraterrestrial Physics (MPE) in Garching near Munich and the Max Planck Institute for Astronomy (MPIA) in Heidelberg provided critical components. After passing through four lenses, a filter, and a beam splitter, it achieves an extraordinarily high contrast. "The requirements for suppressing ghost images are exceeded by a factor of one hundred. The optical design and the precise execution of the optics at MPE and MPIA set new standards for image sharpness and contrast," says Frank Grupp, who led the development of the near-infrared optics at MPE.

MPE is also contributing to research on galaxy evolution. "We have compiled a catalogue of over 70,000 spectroscopic redshifts from various sky surveys and combined it with the Euclid data," explains Christoph Saulder, who led this part of the project. "This catalogue allows for precise distance measurements and the clear identification of numerous galaxies and quasars in Euclid's high-resolution images. It serves as a foundation for a deeper understanding of these objects, their distribution, and their internal properties."

"The new data are also being used to test the techniques for measuring cosmic shear and calibrating redshifts, which will soon be applied to the much larger Euclid data sets to achieve the primary scientific goal - the precision measurement of dark energy," says Hendrik Hildebrandt from Ruhr University Bochum. He leads the key project for measuring cosmic shear and the redshift calibration task force.

Furthermore, scientists at Ludwig Maximilian University (LMU) in Munich have tested methods to identify and characterize galaxy overdensities, a crucial step in tracing the universe's large-scale structure. "The methodologies used to pinpoint galaxy clusters in this task will be key to fully exploiting Euclid's vast dataset, improving cluster identification and contributing to a deeper understanding of cosmic structure formation. At the same time, they help explore previously uncharted regimes in the near-infrared with a statistically significant sample of objects," says LMU scientist Barbara Sartoris.

Likewise, MPIA scientists play leading roles in numerous Euclid studies. They use the data to identify growing supermassive black holes, answer fundamental questions about galaxy evolution, and perform precise photometric measurements of young and old transient celestial objects.

Tracing out the cosmic web in Euclid's deep fields

Euclid has scouted out the three areas in the sky where it will eventually provide the deepest observations of its mission. In just one week of observations and one scan of each region so far, Euclid spotted already 26 million galaxies. The most distant of those are up to 10.5 billion light-years away. The fields span a combined area equivalent to more than 300 times the full Moon.

In order to unravel the mysteries it is designed to explore, Euclid precisely measures the various shapes and the distribution of billions of galaxies with its high-resolution imaging visible instrument (VIS). In contrast, its near-infrared instrument (NISP) is essential for determining galaxy distances and masses.

MPE was responsible for designing and constructing the NISP near-infrared optics. In turn, MPIA carries out crucial tasks for NISP's calibration. "MPIA engineers and scientists are developing and maintaining the mission's entire calibration plan, calibrating and scientifically monitoring the near-infrared camera NISP, performing simulations, and conducting technical analyses such as instrument monitoring," says MPIA's Mischa Schirmer. He is the Euclid mission calibration and NISP calibration scientist.

The new images are a testimony to these efforts and showcase Euclid's capability of mapping hundreds of thousands of galaxies, and start to hint at the large-scale organization of these galaxies in the cosmic web.

Processing large amounts of data using AI

Euclid is expected to capture images of more than 1.5 billion galaxies over six years, sending back around 100 GB of data daily. Such an impressively large dataset creates incredible discovery opportunities, but also poses enormous challenges.

The Euclid consortium has established a European network of nine data centres, including the German Science Data Center (SDC-DE) at MPE. It is equipped with 7,000 processors and processes 10% of the data recorded by Euclid. A team of at least ten experts ensures smooth and consistent processing of astronomical imaging data. MPE's Max Fabricius, who leads the SDC-DE, says: "Approximately 100 GB of raw data is processed virtually in real time every day. The demands on photometric precision are enormous and require a completely new approach to the methods used to calibrate the data."

When it comes to searching for, analysing and cataloguing galaxies, the advancement of machine learning algorithms, in combination with thousands of human citizen science volunteers and experts, is playing a critical role. It is a fundamental and necessary tool to fully exploit Euclid's vast dataset. A significant landmark in this effort is the first detailed catalogue of more than 380,000 galaxies, which have been characterized according to features such as spiral arms, central bars, and tidal tails that infer merging galaxies.

This first catalogue released today represents just 0.4% of the total number of galaxies of similar resolution expected to be imaged over Euclid's lifetime. The final catalogue will present the detailed morphology of at least an order of magnitude more galaxies than ever measured before, helping scientists answer questions like how spiral arms form and how supermassive black holes grow.

Artificial intelligence finds gravitational lenses

Light travelling towards us from distant galaxies is bent and distorted by normal and dark matter in the foreground. This effect is called gravitational lensing and is one of Euclid's tools to reveal how dark matter is distributed throughout the universe. 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.

A first catalogue of 500 galaxy-galaxy strong lens candidates is released today, almost all previously unknown. MPIA scientists were involved in gravitational lensing classifications, labelling images with markers according to their probability of being lenses, as input for machine learning. "These AI systems will ultimately be essential for analysing the 200 times larger sky area at the end of the mission. The number of galaxies distorted by lensing will eventually increase to a staggering 100,000, about 100 times more than currently known. Human classification of individual objects will not be possible for this unprecedented dataset," emphasizes Knud Jahnke from MPIA. He is the NISP instrument scientist.

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 statistically analysing large numbers of galaxies. 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.

Background information

• 2000 square degrees: As of 19 March 2025, Euclid has observed about 2000 square degrees, approximately 14% of the total survey area. The three deep fields together comprise 63.1 square degrees.
• Example data sets: Euclid 'quick' releases, such as the one of 19 March, are of selected areas. They are intended to demonstrate the data products 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.
• Status of the research articles: The data release of 19 March 2025 is described in multiple scientific papers that have not yet been through the peer-review process but will be submitted to the journal Astronomy & Astrophysics.
• Euclid: Euclid was launched in July 2023 and started its routine science observations on 14 February 2024. It is a European mission, built and operated by the European Space Agency (ESA), with contributions from its member states and 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. ESA selected Thales Alenia Space as prime contractor for constructing the satellite and its service module, with Airbus Defence and Space chosen to develop the payload module, including the telescope. NASA provided the detectors of the Near-Infrared Spectrometer and Photometer, NISP. Euclid is a medium-class mission in ESA's Cosmic Vision Programme.
• Strong Contribution of German research institutes: From Germany, the Max Planck Institute for Astronomy in Heidelberg, the Max Planck Institute for Extraterrestrial Physics in Garching, the Ludwig Maximilian University in Munich, the University of Bonn, the Ruhr University Bochum, the University of Bielefeld, and the German Space Agency at the German Aerospace Centre (DLR) in Bonn are participating in the Euclid project.

MN, BEU based on the original ESA press release