Mapping the Perseus Galaxy Cluster with XRISM: Gas Kinematic Features and their Implications for Turbulence

TL;DR

This study uses XRISM/Resolve to map the Perseus cluster's gas kinematics out to ~$0.7\,r_{2500}$ with high spatial resolution, combining 2024 PV data and 2025 GO pointings for a total exposure of $745\,\rm ks$. It finds a pronounced eastern high-dispersion region with nonthermal pressure fractions up to $\sim11\%$, and a large-scale dipole bulk velocity along the east-west axis of $\sim\pm200-300\,\rm km\,s^{-1}$, implying merger-driven rotation. The velocity structure function outside the core favors a single, large energy-injection scale of at least a few hundred kpc, consistent with a merger-driven turbulent cascade. The turbulent heating rate remains roughly uniform over $R\gtrsim60$ kpc and yields an estimated dissipation energy $E_{diss}\sim10^{62}-10^{63}$ erg, comparable to the gravitational energy released by mergers, supporting a significant role for turbulence in energy conversion. Hydrodynamic simulations suggest Perseus has experienced at least two energetic mergers since $z\sim1$, with IC310 likely representing the recent perturber and an earlier event producing a northern outer front; this framework aligns with multi-wavelength observations and highlights future high-resolution spectroscopic missions as key to mapping ICM dynamics.

Abstract

In this paper, we present extended gas kinematic maps of the Perseus cluster by combining five new XRISM/Resolve pointings observed in 2025 with four Performance Verification datasets from 2024, totaling 745 ks net exposure. To date, Perseus remains the only cluster that has been extensively mapped out to ~0.7$r_{2500}$ by XRISM/Resolve, while simultaneously offering sufficient spatial resolution to resolve gaseous substructures driven by mergers and AGN feedback. Our observations cover multiple radial directions and a broad dynamical range, enabling us to characterize the intracluster medium kinematics up to the scale of ~500 kpc. In the measurements, we detect high velocity dispersions ($\simeq$300 km/s) in the eastern region of the cluster, corresponding to a nonthermal pressure fraction of $\simeq$7-13%. The velocity field outside the AGN-dominant region can be effectively described by a single, large-scale kinematic driver based on the velocity structure function, which statistically favors an energy injection scale of at least a few hundred kpc. The estimated turbulent dissipation energy is comparable to the gravitational potential energy released by a recent merger, implying a significant role of turbulent cascade in the merger energy conversion. In the bulk velocity field, we observe a dipole-like pattern along the east-west direction with an amplitude of $\simeq\pm$200-300 km/s, indicating rotational motions induced by the recent merger event. This feature constrains the viewing direction to ~30$^\circ$-50$^\circ$ relative to the normal of the merger plane. Our hydrodynamic simulations suggest that Perseus has experienced at least two energetic mergers since redshift z~1, the latest associated with the radio galaxy IC310. This study showcases exciting scientific opportunities for future missions with high-resolution spectroscopic capabilities (e.g., HUBS, LEM, and NewAthena).

Figures (17)

• Figure 1: XRISM/Resolve pointings used in this work, labeled with their region names. The background shows the XMM-Newton X-ray surface brightness in the $0.5-3.5{\rm \,keV}$ band Churazov2025. White arrows indicate the sloshing cold fronts identified in the system Walker2018. The pointings E, NE, and N (calibration), together with M3 and O3 (GO cycle 1 observations), are newly reported in this work, marked in yellow (see Section \ref{['sec:obs']}).
• Figure 2: Fe He$\alpha$ and Ly$\alpha$ lines in the E region with best-fit 1T and 2T models. The spectrum suggests the presence of an additional, more redshifted, hotter component, which requires confirmation by deeper observations. Residuals normalized by the statistical errors, i.e., (data-model)/error, are displayed in the lower panel (see Section \ref{['sec:obs']} and Appendix \ref{['sec:appendix:obs:e2comp']}).
• Figure 3: The best-fit gas bulk velocity (left) and velocity dispersion (right) maps of the Perseus cluster from nine XRISM/Resolve pointings. The maps are centered on the Perseus center (RA=49.9507, DEC=41.5117). The N and NE regions show the E+NE joint fitting result assuming a single temperature model. The color patches indicate the sky areas that contribute $50\%$ of the photons for the corresponding sub-regions based on raytracing simulations. The measurement uncertainties are shown in Fig. \ref{['fig:contours']}. The Chandra X-ray residual is overlaid in the background highlighting the inner sloshing spiral and the eastern X-ray surface brightness excess, with black circles marking radii of 100, 200, and $300{\rm \,kpc}$. The bulk velocity distribution, in the rest frame of the central ICM ($z_{\rm icm}=0.017628$), shows an (asymmetric) dipole-like pattern, revealing a rotational motion of the ICM (see Section \ref{['fig:vmap']}).
• Figure 4: 2D confidence contours (1, 2, and 3$\sigma$) of the bulk velocity and velocity dispersion measurements of our new regions (calibration and GO pointings). The red contours indicate a joint fit between the E and NE regions (see Fig. \ref{['fig:contours_appendix']} for their individual fits). The crosses mark the best-fit parameters and their $1\sigma$ uncertainties, including also all PV data (ten sub-regions; see Section \ref{['sec:map']}). The pink cross indicates the cold ICM component in the 2T model for region E (see Section \ref{['sec:obs']}).
• Figure 5: Nonthermal pressure fraction profile of the Perseus cluster, compared with A2029 (grey points; XRISM2025_A2029_Outer). Colors follow the same scheme as in Fig. \ref{['fig:contours']} for Perseus. The shaded bands show numerical predictions from TNG cosmological simulations for two cluster subsamples: (i) the 31 most relaxed clusters from the TNG-300 simulation suite (yellow) and (ii) 30 Perseus-like massive, cool-core clusters from TNG-Cluster suite (pink). The bands represent the 10th-90th percentile range of the $f_{\rm nth}$ distribution within each sample. The inner AGN-dominant region, as well as the regions overlapping the eastern X-ray surface brightness excess (E+NE and O3), show high $f_{\rm nth}$ ($>7\%$; see Section \ref{['sec:velocity:fnth']}).