The Quiescent Merging Nature of the Coma Cluster Revealed by ICM Velocity Structure

TL;DR

This study uses high-resolution X-ray spectroscopy to reveal a coherent velocity gradient in the Coma cluster's intracluster medium, indicating a past off-axis merger and providing insights into cluster dynamics.

Contribution

The paper presents new XRISM observations that detect a large-scale velocity gradient, offering novel evidence of a past merger in the Coma cluster through ICM velocity structure analysis.

Findings

Detected a 530 km/s velocity gradient across Coma

Found uniform thermodynamic properties indicating mild disturbances

Provided evidence for an off-axis merger event

Abstract

The hot gas permeating galaxy clusters-the intracluster medium (ICM)-is a key tracer of their assembly history and internal dynamics. Understanding the motion of this gas provides critical insight into processes such as mergers, turbulence, and energy dissipation in the largest gravitationally bound structures in the Universe. The Coma cluster is a nearby, massive system long suspected to be dynamically disturbed. Previous high-resolution X-ray spectroscopy with the XRISM mission revealed bulk motions in the cluster core and southern regions. Here we present new XRISM Resolve observations of a northern region in Coma, which reveal a coherent velocity gradient of nearly $530 km/s across the cluster from south to north. We find that the hot gas in this northern region exhibits modest line-of-sight motions and uniform thermodynamic properties, indicating relatively mild local disturbances. The consistent levels of turbulence throughout the cluster suggest that the energy from a past merger has been distributed on large scales. These findings provide compelling evidence for an off-axis merger event and demonstrate how high-resolution X-ray spectroscopy can uncover subtle dynamical signatures in the ICM, offering important constraints for simulations of cluster evolution.