A Minimalist Merger Interpretation of XRISM's Gas Velocity Measurements in the Coma Cluster

TL;DR

The paper addresses how XRISM's velocity maps of the Coma cluster can be interpreted, demonstrating that an off-axis minor merger in an early phase can drive large-scale bulk motions and a modest core dispersion comparable to observations, with v_core ≈ -500 km s^{-1}, a gradient of ≈500 km s^{-1} over ≈400 kpc, and σ_los ≈ 100–200 km s^{-1}. Using an idealized Arepo simulation of a merger with a mass ratio around 6, initial speed ~500 km s^{-1}, and i ≈ 30–40°, the authors reproduce the key velocity features at t ≈ 1 Gyr after pericenter, including a strong LOS bulk velocity in the core and limited central turbulence. They introduce and apply the velocity-ratio diagnostic, u_los/σ_los, showing that Coma’s measured values (negative and broad) are inconsistent with Gaussian turbulence, consistent with a transient merger-driven state and not fully developed cascade. The results emphasize the need for tailored, cluster-specific simulations to interpret XRISM data and suggest a simple, robust diagnostic for identifying the dominant motion scales in the ICM.

Abstract

The recent microcalorimetric X-ray observations of the Coma cluster by XRISM have sparked active discussion regarding the physical origin of its gas velocity features. Here, we demonstrate that an off-axis minor merger in its early phase $-$ when the infalling subhalo is near its primary apocenter and the stripped tail is not yet mixed with the main cluster atmosphere $-$ can drive intracluster gas motions generally consistent with the XRISM results. These include a pronounced velocity gradient and an approximately uniform velocity dispersion of $\simeq100-200\,\rm{km\,s^{-1}}$ in the cluster core. Our merger scenario was originally suggested in Lyskova et al. (2019) to reproduce the major X-ray morphological features of Coma. In addition, we introduce a simple and robust diagnostic of intracluster gas motions based on the ratio of the line-of-sight velocity to the velocity dispersion.

Figures (6)

• Figure 1: An off-axis minor merger between two non–cool-core clusters with a merger mass ratio of $\xi_0=6$. The panels show the simulated X-ray surface brightness ($0.5-8{\rm \,keV}$), X-ray–weighted temperature, LOS bulk velocity, and velocity dispersion at $t=1.0{\rm \,Gyr}$ after the pericentric passage, viewed at an inclination angle of $i\simeq40^\circ$. Black contours in the left panels indicate the total projected mass distribution. The dark-red line marks the trajectory of the subhalo. Zoom-in views of the gas velocity distributions in the regions outlined by the black boxes are shown in Fig. \ref{['fig:merger_proj_zoom']}. This simulation well resembles Coma in X-ray observations (see Section \ref{['sec:merger']}).
• Figure 2: Zoom-in views of the gas velocity distributions in the core of the main cluster (top) and the infalling group (bottom). Black contours indicate the X-ray surface brightness distribution. The small white box in the bottom-right corner marks the size of the XRISM/Resolve's full field-of-view. The simulation reproduces key velocity features in the core of Coma (see Section \ref{['sec:merger']}).
• Figure 3: PDF of the velocity ratio $u_{\rm los}/\sigma_{\rm los}$ obtained in our idealized merger simulation. The solid lines show its time evolution within $r<250{\rm \,kpc}$ of the main cluster. The dashed purple line shows the result from a merger with a larger mass ratio $\xi=10$ (see the bottom panels in Fig. \ref{['fig:merger_proj_params']}), indicating that the velocity ratio is relatively insensitive to the absolute velocity amplitude. The shaded grey regions indicate the three XRISM measurements in Coma, supporting our argument that merger-driven, large-scale bulk motions dominate the observed velocity fields in Coma (see Section \ref{['sec:merger']}).
• Figure 4: Velocity-ratio PDFs (bottom) derived from Gaussian random fields with different underlying energy power spectra. The adopted $E(k)$ are illustrated in the top panel with $(\ell_{\rm inj},\,\alpha,\,\ell_{\rm diss})=$$(1{\rm \,Mpc},\,-5/3,\,0.1{\rm \,kpc})$, $(1{\rm \,Mpc},\,-5/3,\,300{\rm \,kpc})$, $(1{\rm \,Mpc},\,-6,\,0.1{\rm \,kpc})$, and $(0.1{\rm \,Mpc},\,-5/3,\,0.1{\rm \,kpc})$ for the purple, yellow, green, and blue lines, respectively. The vertical band in the top panel marks the approximate range of $\ell_{\rm eff}$ in the Coma gas core. The bands in the bottom panel indicate the three XRISM measurements in Coma. These measurements deviate significantly from the predictions, indicating that the cluster is in a merger-driven, transient phase, where velocity distributions depart from Gaussianity (see Section \ref{['sec:discussion:scale']}).
• Figure 5: Same as Fig. \ref{['fig:merger_proj']}, but for simulations with different merger mass ratios and initial impact parameters (both with $V_0=500{\rm \,km\,s^{-1}}$). For simplicity, we adopt the same inclination angle for the projection as in Fig. \ref{['fig:merger_proj']}. These examples illustrate that slightly varying the merger parameters produces broadly similar X-ray morphologies and velocity patterns.