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

TL;DR

This study uses XRISM Resolve to map the velocity field of the Coma cluster's ICM, combining a new northern-region pointing with prior core and southern measurements. A single-temperature spectral model suffices, revealing a coherent velocity gradient of about $530\ \mathrm{km\,s^{-1}}$ across hundreds of kiloparsecs and subsonic turbulence with $\mathcal{M}_{\rm 3D} = 0.197 \pm 0.046$, while the turbulent-to-thermal energy fraction is $E_{\rm turb}/E_{\rm therm} = 0.032 \pm 0.015$ and the kinetic pressure fraction is $P_{\rm turb}/P_{\rm tot} = 0.021 \pm 0.010$. The ICM bulk motion relative to local galaxies is significant, $\Delta V = -711^{+68}_{-72}$ km s$^{-1}$, consistent with a dynamically distinct gas substructure. Collectively, the results support an off-axis merger scenario that drives large-scale bulk flows while maintaining near-uniform thermodynamics on small scales, underscoring XRISM Resolve's power to constrain cluster evolution and dynamics.

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.

Figures (4)

• Figure 1: Line-of-sight bulk velocities ($v_{\rm bulk}$) for the Coma cluster: northern region (yellow) and center/south (white/green; xri25c). Values are shown for the full field of view and quadrants, overlaid on the XMM-Newton X-ray image san20. Quadrant points include mean uncertainties.
• Figure 2: Velocity maps obtained for the Coma cluster. Top panel: Bulk velocities ($v_{\rm bulk}$) obtained from the XRISM Resolve spectra. Bottom panel: Turbulent velocities ($\sigma_{v}$). The Chandra contours for the broadband energy range (0.5-7 keV) are included.
• Figure 3: Spatial distribution of Coma ICM line-of-sight velocities. XRISM Resolve pointings (north, center, south) are divided into quadrants (NE, NW, SE, SW) and color-coded by $v_{\rm bulk}$. Smoothed X-ray contours show gas density, the blue line indicates the best-fit velocity gradient (shaded region = uncertainty), and the dashed red line marks the cluster major axis (PA = 65$^\circ$). The plot highlights a coherent large-scale velocity shear slightly misaligned with the cluster elongation.
• Figure 4: XRISM Resolve spectrum zoomed on the strongest Fe K-shell lines. He-like triplet components are shown: resonance ($w$), intercombination ($x,y$), and forbidden ($z$); Fe XXVI Ly$\alpha$ has $\alpha_1$ and $\alpha_2$ transitions. The spectrum is rebinned for clarity. Lower panels show fit residuals for bapec (red), 2-bapec (blue), and blognorm (green). Minor deviations in Fe XXVI $\alpha_2$ and Fe XXV intercombination lines resemble those seen in A2029 and Centaurus xri25axri25c.