THE COSMIC WEB IN THE FIRST 63 SQUARE DEGREES OF THE EUCLID SURVEY: FIRST INDICATIONS OF ITS IMPACT ON GALAXIES IN FILAMENTS AND CLUSTERS
On March 19, 2025, with the ‘Q1 release’ the first 63 square degrees of the Euclid survey are released to the world. 34 preprints describe how these data were processed and present the first scientific results from the Euclid survey. The IAP leads both the team behind the automatic data processing pipeline for Euclid’s VIS camera and two scientific papers that offer an exciting glimpse of the revolutionary science that Euclid will achieve. These papers provide new evidence of how galaxies grow inside the filamentary structure of the cosmic web.
Dark matter provides the scaffolding on which are built the Universes’ largest structures; dark energy is responsible for the accelerated expansion of the Universe. The ultimate aim of Euclid is to probe the nature of these two mysterious constituents of the Universe. To do this, Euclid is surveying the entire extragalactic sky using two instruments, VIS and NISP. Data from these instruments provide positions and shapes for billions of galaxies. From these shapes, Euclid measures the tiny distortions induced by dark matter along the line of sight; from the positions, the distribution of galaxies on the largest scales. These two effects, combined, provide a sensitive test of the characteristics of dark matter and dark energy.
Since its launch in 2023, Euclid has already surveyed several thousands of square degrees. To make the first cosmological measurements will require further rigorous testing. The first full data release for cosmology is planned for public release in July 2026. But already, thanks to the excellent quality of the Euclid data combined with an automatic pipeline that produces fully processed data, many long-standing unresolved scientific questions can be addressed with even a small quantity of the Euclid VIS and NISP data. Led from the IAP for more than a decade, a team of scientists and engineers have perfected a data processing system which takes raw data from the VIS camera and produces fully calibrated images ready for science. The Q1 release is designed to provide a glimpse of the kinds of scientific questions beyond cosmology that Euclid can address, together with making these fully processed data available to the scientific community so that they can begin their own independent scientific research.
The ‘cosmic web’ is the immense filamentary structure of dark matter and galaxies that fills the Universe in our standard model of cosmology. Galaxies (mostly) form in this cosmic web at the centres of massive haloes of dark matter. As shown in the wonderful Euclid Q1 data over an unprecedented volume at this high resolution, galaxies are very diverse in sizes, colours and shapes. What drives this diversity is still a long-standing debate. In particular, astrophysicists try to understand what is the role of the cosmic web environment in galaxy evolution. Just as in psychology social interactions influence personality, theoreticians expect that the environment around galaxies shape their morphologies! But conducting this analysis has always been difficult in practice, especially when trying to observe galaxies at large distances, because it requires both extracting the structure of the cosmic web over a large volume of the universe, and finely measuring the morphology of galaxies.
Figure 1: A hundred of likely cosmic filament distributions in projection are superimposed on the Euclid image in part of the EDF-F field. The panels on the left and bottom illustrate two of the measurements carried out as part of the Q1 publications.
Credit: Background picture: ESASky (from https://sky.esa.int/esasky); Cosmic Filaments: Clotilde Laigle, IAP/ Sébastien Derriere, CDS, Observatoire astronomique de Strasbourg.
Euclid’s VIS images are an ideal instrument to make a step forward. Thanks to the large field of the VIS instrument, it is possible to image hundreds of thousands of galaxies at once. And because of the excellent resolution of VIS, we can precisely measure the shape of each galaxy. Two preprints published as part of the Q1 release, led by scientists at the IAP, Clotilde Laigle and Celine Gouin, use these unique data to investigate galaxy formation inside the cosmic web.
If the cosmic web environment is important for the fate of the galaxies, we should observe variations in their shapes depending on their position in the cosmic web. This is precisely what Laigle et al. detect: after mapping the cosmic web’s filamentary structure, they find that elliptical galaxies are closer to the filament spine than disk galaxies. In addition, they detect a tendency for the shape of these elliptical galaxies to align with the filaments, a signal that we expect from the theory if these galaxies have merged together when flowing along the same filaments. This result demonstrates that Euclid has great potential to make a major contribution to building a coherent model of galaxy formation in the cosmic web.
Figure 2: The mass–connectivity relation measured in Euclid Q1 data (in black) compared with, simulations from IllustrisTNG in red and HorizonAGN in orange, and observations from CFHTLS in blue and COSMOS in green.
Credit: Euclid Collaboration, C. Gouin et al. (2025).
The second paper, led by Celine Gouin, takes a different approach. They use existing catalogues of massive objects — galaxy clusters — to investigate how they are connected to filaments. By using the cluster connectivity, i.e. the number of filaments connected to clusters, they explore the influence of large-scale cosmic web environment on cluster properties. Figure 2. shows how the ‘connectivity’ of these objects depends on their mass. They find that the more massive an object is, the more connected it is to the cosmic web. This implies that the most massive objects form at the intersection of many filaments of the cosmic web. Even more intriguing, we discover that clusters predominantly made up of elliptical galaxies are more strongly connected to filaments than those dominated by disk galaxies. These findings suggest that the cosmic web doesn’t just connect to clusters—it plays a crucial role in their properties. With upcoming observations from Euclid, which will dramatically increase the number of clusters studied and provide a more precise map of the cosmic web, we’re excited to see where this research leads in advancing our understanding of the cosmic environment that drives cluster formation and evolution!
What is most remarkable is that these measurements were made with less than 1% of the total area of Euclid. They provide clear evidence of how the cosmic web plays a key role in galaxy formation and evolution in the distant Universe, expanding previous analyses in the nearby Universe or on small volumes. They provide an exciting glimpse of how high-resolution Euclid images will transform our understanding of galaxy formation and evolution across cosmic time.
Links
Euclid France Press release (in French): Nouveaux résultats scientifiques et nouvelles données du télescope spatial Euclid
The ESA Press release: Euclid opens data treasure trove, offers glimpse of deep fields.
The CNRS-INSU Press release (in French): “Quick-release 1” : le télescope spatial Euclid dévoile sa première moisson d’images de l’Univers.
Q1 Data Access: Euclid Q1 Data Access
Preprints
The role of cosmic connectivity in shaping galaxy clusters: Euclid collaboration: Gouin et al.
Galaxy shapes and alignments in the cosmic web: Euclid collaboration: Laigle et al.
VIS processing and data products: Euclid collaboration: McCracken et al.
Writing and contacts
- Céline Gouin
Institut d’astrophysique de Paris, CNRS, Sorbonne Université
celine [dot] gouin [at] iap [dot] fr - Clotilde Laigle
Institut d’astrophysique de Paris, CNRS, Sorbonne Université
clotilde [dot] laigle [at] iap [dot] fr - Henry Joy McCracken
Institut d’astrophysique de Paris, CNRS, Sorbonne Université
hjmcc [at] iap [dot] fr
Layout: Jean Mouette
March 2025