# Observations of a Pre-Merger Shock in Colliding Clusters of Galaxies

## TL;DR

This paper reports the discovery of a unique pre-merger shock in a galaxy cluster pair, providing new insights into early cluster collision dynamics and the thermal history of large-scale structures.

## Contribution

It presents the first observation of a pre-merger shock propagating outward along the equatorial plane, an early-phase phenomenon not previously documented.

## Key findings

- Discovery of a shock in a pre-merger galaxy cluster pair.
- Shock propagates outward along the equatorial plane, unlike typical merger shocks.
- Supports models predicting shock energy dissipation outside clusters.

## Abstract

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.