The Past History of Galaxy Clusters told by their present neighbors
  Galaxy 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 neighbor distribution of cluster-size halos and
the latter mass assembly history. From the Big Multidark simulation, we extract
two large samples of random halos 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 halos to 200
Virgo-like halos extracted from simulations that resemble the local Universe.
The number of neighbors at different cluster-centric distances permits
discriminating between clusters with different mass accretion histories.
Similarly to Virgo-like halos, clusters with numerous neighbors within a
distance of about 2 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. On the contrary, clusters with few neighbors share an opposite trend:
from passive to active assembly histories. Additionally, clusters with massive
companions within about 4 times their virial radius tend to have recent active
merging histories. Therefore, the radial distribution of cluster neighbors
provides invaluable insights into the past history of these objects.
