The past history of galaxy clusters told by their present neighbours

International audienceGalaxy clusters can play a key role in modern cosmology, provided their evolution is properly understood. However, observed clusters give us only a single timeframe of their dynamical state. Therefore, finding present observable data of clusters that are well correlated to their assembly history constitutes an inestimable tool for cosmology. Former studies correlating environmental descriptors of clusters to their formation history are dominated by halo mass–environment relations. This paper presents a mass-free correlation between the present neighbour distribution of cluster-size haloes and the latter mass assembly history. From the Big Multidark simulation, we extract two large samples of random haloes with masses ranging from Virgo to Coma cluster sizes. Additionally, to find the main environmental culprit for the formation history of the Virgo cluster, we compare the Virgo-size haloes to 200 Virgo-like haloes extracted from simulations that resemble the local Universe. The number of neighbours at different cluster-centric distances permits discriminating between clusters with different mass accretion histories. Similarly to Virgo-like haloes, clusters with numerous neighbours within a distance of about two times their virial radius experience a transition at z ≈ 1 between an active period of mass accretion, relative to the mean, and a quiet history. In contrary, clusters with few neighbours share an opposite trend: from passive to active assembly histories. Additionally, clusters with massive companions within about four times their virial radius tend to have recent active merging histories. Therefore, the radial distribution of cluster neighbours provides invaluable insights into the past history of these objects