​Galaxy clusters are composed of dark matter (85%), hot gas (12%), and only a small percentage of stellar matter (galaxies). The process of cluster formation is therefore dominated by the gravitational collapse of dark matter. The movement of the gravitational collapse carries the gas and galaxies, and heats the gas while ionizing it. How can this process be observed? The hot gas emits X-rays, and can therefore be observed by X-ray space observatories.

An alternative solution is to use the cosmic microwave background (CMB, the oldest light in the universe), in which the universe has been immersed since 380,000 years after the Big Bang. The interaction between these photons and the high-energy electrons of the intra-cluster gas causes distortions of the spectrum of the CMB, directed toward these clusters. These very low distortions can now be measured at high spatial resolution thanks to the new IRAM Kinetic Inductive Detector Array (NIKA).

Recordings by NIKA at Pico Veleta, near Granada, Spain, provide the first accurate mapping of the velocity of hot gas within clusters located at a distance of 5 billion light-years. They provide a new approach for the study of intra- and inter-cluster collisions. These are responsible for the most energetic events in the universe since the Big Bang and played a role in the organization of the large structures of the "cosmic web" observed today.

Thanks to a new camera called NIKA2, with even higher sensitivity compared to NIKA, the researchers will be able to study about 50 clusters located between 5 and 7 billion light-years and deepen their understanding of the formation of the large structures of the universe.

This research was carried out in collaboration with the Joseph-Louis Lagrange laboratory in Nice (Côte d'Azur Observatory, CNRS, Université Côte d'Azur).