Letter to the Editor

Coherent bulk motions in a weakly turbulent merging Coma cluster

¹ Max-Planck-Institut für extraterrestrische Physik Gießenbachstraße 1 85748 Garching, Germany
² Institute for Frontiers in Astronomy and Astrophysics, Beijing Normal University Beijing 102206, China
³ Institute of Astronomy Madingley Road Cambridge CB3 0HA, UK
⁴ INAF – IASF Palermo Via U. La Malfa 153 I-90146 Palermo, Italy
⁵ Department of Astronomy, University of Geneva Ch. d’Ecogia 16 CH-1290 Versoix, Switzerland
⁶ Department of Physics and Astronomy, University of Alabama in Huntsville Huntsville AL 35899, USA

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 11 November 2025
Accepted: 31 January 2026

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.