Complete census of massive slow rotators in ten large galaxy clusters
  Galaxy interactions leave imprints in the motions of their stars, and so
observing the two-dimensional stellar kinematics allows us to uncover their
formation process. Slow rotators, which have stellar orbits dominated by random
motions, are thought to be the fossil relics of a sequence of multiple gas-poor
mergers, in an environment where the cold gas required to form new stars is
nearly absent. Indeed, observations of a handful of nearby galaxy clusters have
indicated that slow rotators are preferentially found in the gas-poor, dense
cores of clusters, which themselves must form by merging of smaller groups.
However, the generality of this result and connection between kinematics and
environment is currently unclear, as recent studies have suggested that, at
given stellar mass, the environment does not influence the formation of slow
rotators. Here we address this issue by combining a careful quality-assessed
sample selection with two-dimensional stellar kinematics from a large galaxy
survey and a novel photometric classification approach where kinematics are
unavailable. We obtain the first complete census of the location of massive
slow rotators in ten large clusters: in all cases, slow rotators are extremely
rare and generally trace the clusters density peaks. This result unambiguously
establishes that massive slow rotators are the relics of violent hierarchical
cluster formation.
