Abstract

Galaxy clusters are powerful cosmological probes, tracing the matter content and large-scale structure of the Universe. Observed across the electromagnetic spectrum, they are dominated by dark matter with baryons in hot gas and galaxies. Recent and forthcoming surveys (Planck, eROSITA, Euclid, LSST) are delivering large cluster samples, enabling precision cosmology. However, cosmological constraints are currently limited by uncertainties in mass–observable scaling relations, which require improved modeling through multi-wavelength observations and simulations. We propose a two-day workshop to bring together the French galaxy cluster community to share results, discuss methodologies, and foster collaborations. Particular emphasis will be placed on the participation of students and postdoctoral researchers.

Scientific justification

Galaxy clusters are powerful cosmological probes, tracing the matter content of the Universe and providing insights on the state of expansion and the clustering of matter in the universe. Composed mainly of dark matter, with baryons in hot gas and galaxies, clusters are observed across the electromagnetic spectrum. Recent surveys have produced catalogues of hundreds to thousands of clusters (e.g. PLANCK, DES, SPT, eROSITA), and ongoing and future experiments will soon deliver thousands over a wide redshift range (e.g. Euclid and LSST), marking the transition to precision cluster cosmology.

Cluster observables—such as number counts, clustering, gas fraction, and mass profile—depend critically on cluster mass and redshift, which must be inferred from wavelength-dependent mass proxies. While clusters yield tight cosmological constraints, uncertainties in mass–observable scaling relations remain the dominant limitation. Improving cluster physics through multi-wavelength observations and hydrodynamical simulations is therefore essential to control systematics and refine cosmological analyses.

Achieving precision cluster cosmology requires a coordinated effort combining data analysis and simulations, which is particularly timely given the wealth of current data and the advent of Stage IV experiments. The strong involvement of the French galaxy cluster community in major international collaborations provides a valuable opportunity to consolidate and enhance this expertise at the national level.

We organize a two-day workshop aimed at bringing together the French galaxy cluster community to present ongoing contributions to major survey programs and to exchange on the diverse scientific and methodological aspects of galaxy cluster research developed across different groups. The meeting will foster discussion, coordination, and new collaborations within the community, strengthening its national cohesion and international visibility.

A key objective of the workshop is to actively involve early-career researchers. Students and postdoctoral fellows will be especially encouraged to present their work, providing them with visibility, feedback, and networking opportunities.

Preliminary program

– X-ray galaxy cluster observations (CHEXMATE) – G. Pratt, M. Pierre
– SZ clusters (SPT, ACT & NIKA2 programs) – J.P. Melin, F. Mayet, E. Pointecouteau
– Simulations of galaxy clusters – A. Le Brun, F. Tornatore, Y. Dubois
– Shear Lensing – S. Pires, S. Farrens
– Strong Lensing Clusters – M. Limousin, E. Jullo, A. Niemec
– Cluster cosmology (LSST preparation) – C. Combet