Exploring the dynamics of the Coma galaxy cluster by mapping its X-ray emission line profiles with XRISM

Abstract The intracluster medium (ICM) in merging galaxy clusters exhibits turbulence and bulk flows. Unraveling these components is crucial not only for elucidating the geometry of the cluster mergers, but also for understanding the physics of magnetic field amplification and relativistic particle acceleration. XRISM/Resolve data for two $3^{\prime }\times 3^{\prime }$ fields in the core of the Coma cluster reveals that the ICM in the central field moves with $\Delta cz = -430$ km s$^{-1}$ relative to the cluster galaxy average, while that in the southern field moves with $\Delta cz = -730$ km s$^{-1}$ (see XRISM Collaboration et al. 2025, ApJ, 985, L20; hereinafter “Paper I”). In this paper, we perform a more detailed analysis of these data sets to search for non-Gaussian features in the Fe K line complex profiles. In the spectra from the north-west quadrant of the central field, in addition to the main and redshifted ICM components ($\Delta cz = -6$ km s$^{-1}$) reported in Paper I, we find evidence of another, blueshifted, component, moving with $\Delta cz = -1274$ km s$^{-1}$. For a systematic search for other significant velocity components, we perform a bias-free $3$ eV step multi-component fit to the Resolve full-array spectra from the central and southern fields. This search uncovers another redshifted component in the southern field, moving with $\Delta cz \sim +1250$ km s$^{-1}$. We estimate the energy densities of the ICM turbulence and bulk motion to be similar to each other and several times greater than the energy density of the cluster’s $B\sim 5\, \mu$G magnetic field.