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The main goal of the M2C project was to test the standard Λ-Cold Dark Matter (ΛCDM) scenario for the formation of collapsed structures, using the dark matter profiles in the most massive galaxy clusters, and their evolution, as cosmic laboratories. In parallel, the project assessed the thermodynamic behaviour of the baryons as they collect in the halo potential. The project built on the advent of cluster detection via the Sunyaev-Zeldovich (SZ) effect in the sub-mm band and required the assembly of a very large data set of observational and theoretical data.
We have developed a completely new cluster detection tool, combining SZ and X-ray survey data, and we published the all-sky ComPRASS catalog obtained with it. Combined detection was identified as one of the main challenges of the project, and we demonstrated that it is more performant than single survey detection. Furthermore, we extended the method to the combination of inhomogeneous ground-based and space SZ observations.
We identified new massive clusters at high redshift, from pure SZ detection, by improving the validation procedure and candidate preselection well beyond what was originally envisaged in the original proposal. As a result, we have assembled and homogenised a rich multi-wavelength data set on X-ray, SZ and X-ray+SZ detected clusters, which we make available to the community through the M2C database.
Thanks to the precision and redshift leverage of our measurement of the total mass profiles, we have brought to light, and quantified, the key dependence of the dark matter profile shape on the dynamical state. It proved to be more important than the dependence on redshift or mass. In parallel, we performed unique, large-volume, high resolution cosmological simulations, to provide, for the first time, simulated samples with similar selection characteristics to those available to observers.
Concerning the baryonic component, we focussed on aspects uniquely feasible with the project sample, owing to its unique selection strategy and high quality follow-up. We showed that the population of massive clusters is dominated by disturbed objects, or objects without a cool core, at all redshifts, with no significant evolution of either the core or large-scale properties. We put new constraints on the timescale and location of star formation quenching in clusters, and we showed that galaxy-merging is likely the dominant mechanism of AGN triggering in cluster outskirts.
M2C, and the other projects in which we were involved, underlined that a new local anchor is needed for tracking population changes. In this connection, an unexpected and major outcome of the project was the award of the Multi-year heritage project with XMM-Newton (PI M. Arnaud and S. Ettori). This emphasizes the importance of extending the general philosophy of M2C to the study of the baryonic content.