The plasma physics of galaxy clusters in a cosmological context: connecting micro and macro scales

Project Information

CosmoClusterPlasmas

Grant agreement ID: 101106080

DOI

10.3030/101106080

Project closed

EC signature date 20 March 2023

Start date 1 September 2023

End date 31 August 2025

Funded under

• Marie Skłodowska-Curie Actions (MSCA)

Total cost

No data

EU contribution

€ 230 774,40

Coordinated by

KOBENHAVNS UNIVERSITET
Denmark

Periodic Reporting for period 1 - CosmoClusterPlasmas (The plasma physics of galaxy clusters in a cosmological context: connecting micro and macro scales)

Reporting period: 2023-09-01 to 2025-08-31

Summary of the context and overall objectives of the project

Galaxy clusters are the largest gravitationally bound structures in the Universe and play a central role in modern cosmology. They are used to trace the growth of cosmic structure, to test theories of gravity, and to constrain the fundamental properties of dark matter and dark energy. A key component of galaxy clusters is the intracluster medium: a vast reservoir of extremely hot, diffuse plasma that emits X-rays and contains most of the normal (baryonic) matter in clusters. The physical state of this plasma encodes the formation history of clusters and the complex processes—such as turbulence, heating, cooling, and mergers—that shape them over billions of years.
Until recently, direct measurements of gas motions inside galaxy clusters were beyond observational reach. This has now changed with the successful launch of the XRISM X-ray telescope, which is delivering high-precision measurements of intracluster gas velocities and turbulence. These observations are opening a new era in X-ray astronomy, allowing scientists to probe cluster dynamics and plasma processes under extreme conditions that cannot be reproduced in laboratories on Earth. However, exploiting this observational breakthrough requires equally advanced theoretical models. A major limitation of traditional cosmological simulations is the assumption that the intracluster medium behaves as a fully collisional fluid. In reality, the plasma in galaxy clusters is only weakly collisional, meaning that particle interactions, transport processes, and instabilities differ fundamentally from those in ordinary fluids. Neglecting these effects limits the accuracy of theoretical predictions and hampers the interpretation of high-quality X-ray data.
The objective of this project was to overcome this limitation by developing and applying the first cosmological galaxy-cluster simulations that move beyond the standard collisional assumption. By incorporating the physics of weakly collisional plasmas into state-of-the-art numerical simulations, the project aimed to produce realistic predictions for turbulence, gas motions, and energy transport in galaxy clusters.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

The project focused on advancing the theoretical understanding of the intracluster medium (ICM) through large-scale numerical simulations and the development of new analysis tools. The core activities included the generation of high-resolution cosmological simulations of galaxy clusters, the implementation of improved numerical techniques, and the detailed analysis of turbulence, magnetic fields, and plasma processes in the ICM.

A major technical achievement was the construction of a new parent cosmological simulation covering a volume of 1 Gpc/h, from which 272 massive galaxy clusters were identified. From this sample, 24 clusters were re-simulated using high-resolution “zoom-in” techniques at IllustrisTNG300 resolution while ensuring contamination-free initial conditions. In addition, the a very massive cluster was re-simulated at IllustrisTNG100 resolution, resulting in the highest-mass, highest-resolution cosmological galaxy cluster simulation currently available with the IllustrisTNG model galaxy formation model. These simulations were performed using the AREPO2 code, enabling improved accuracy and scalability to the unprecedented numerical resolution for massive galaxy clusters.

The project also delivered several important software developments. A new initial-condition generation method was created to eliminate numerical contamination, addressing a limitation of some other cluster zoom simulations. A dedicated turbulence-diagnostics code was developed to separate cluster turbulence from bulk motions. In addition, a modern visualisation and analysis framework was produced to enable efficient exploration of the very large simulation datasets.

Scientifically, the simulations were used to investigate turbulence generation, magnetic-field amplification, and viscous heating in galaxy clusters. The work provided new insight into how plasma microphysics and heat transport affect turbulent motions, particularly in cluster outskirts. A detailed post-processing analysis of viscous (Braginskii) heating in MHD simulations indicated that while viscous heating is not sufficient to maintain thermal balance alone, it represents a non-negligible contribution. The project also demonstrated that magnetic fields are primarily amplified within galaxies at early times, implying that detailed microphysical plasma effects are not essential for capturing the large-scale magnetic dynamo in clusters.

Overall, the project produced a unique, high-resolution simulation suite, new numerical tools, and a set of quantitative predictions that can be compared with X-ray and radio observations of galaxy clusters.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

The project delivered results that go significantly beyond the current state of the art in both numerical methodology and physical understanding of galaxy clusters.

A key advance is the production of the highest-resolution, fully contamination-free cosmological simulations of massive galaxy clusters to date. Existing simulation suites often suffer from numerical contamination or insufficient resolution in cluster outskirts, limiting their reliability. By developing a new initial-condition methodology and exploiting the capabilities of AREPO2, the project demonstrated that extremely high-resolution simulations of massive clusters are feasible and scientifically robust. This represents a step change in the fidelity with which the dynamics, turbulence, and magnetic fields of the intracluster medium can be modelled.

Another major advance concerns the diagnosis of turbulence in cosmological simulations. The project identified fundamental mathematical and conceptual shortcomings in widely used turbulence-analysis techniques and developed an improved diagnostic framework tailored to diagnose turbulence in cosmological simulations. This new approach enables a more well defined separation of turbulent motions from large-scale flows and provides well defined measures of turbulence strength. This improved understanding is essential for comparing with X-ray measurements of gas motions.

The project’s results have broad relevance for several scientific communities, including X-ray and radio astronomers and plasma physicists. The large simulation campaign, which required approximately 30 million CPU-hours and produced around 200 TB of data, constitutes a major legacy outcome. It already underpins multiple scientific publications and is expected to support many further studies in the coming years, serving as a cornerstone dataset for the community working with the AREPO code.

In addition, the project produced three software outputs with strong potential for further uptake: a scalable visualisation and analysis framework for large cosmological datasets, an advanced turbulence-diagnostics tool applicable beyond galaxy clusters, and key code developments that enable high-resolution cluster simulations. Ensuring long-term impact will require continued curation of the simulation data, further methodological development, and close interaction with observational programmes.

A large-scale view of a galaxy cluster simulation.