Observations of a Pre-Merger Shock in Colliding Clusters of Galaxies
  Clusters of galaxies are the largest known gravitationally-bound structures
in the Universe. When clusters collide, they create merger shocks on
cosmological scales, which transform most of the kinetic energy carried by the
cluster gaseous halos into heat. Observations of merger shocks provide key
information of the merger dynamics, and enable insights into the formation and
thermal history of the large-scale structures. Nearly all of the merger shocks
are found in systems where the clusters have already collided, knowledge of
shocks in the pre-merger phase is a crucial missing ingredient. Here we report
on the discovery of a unique shock in a cluster pair 1E 2216 and 1E 2215. The
two clusters are observed at an early phase of major merger. Contrary to all
the known merger shocks observed ubiquitously on merger axes, the new shock
propagates outward along the equatorial plane of the merger. This discovery
uncovers an important epoch in the formation of massive clusters, when the
rapid approach of the cluster pair leads to strong compression of gas along the
merger axis. Current theoretical models predict that the bulk of the shock
energy might be dissipated outside the clusters, and eventually turn into heat
of the pristine gas in the circum-cluster space.
