Kinetic structure of the intracluster medium across nearby clusters observed with XRISM

Abstract: XRISM/Resolve is building a sample of galaxy clusters with directly measured ICM gas motions, revealing diverse projected dynamical states. We compile 45 XRISM/Resolve measurements in 19 nearby galaxy clusters and place them on a common, emission-weighted effective line-of-sight scale, $\ell_{\rm eff}$. We compare the line-of-sight velocity dispersion $σ<em>v$, bulk velocity amplitude $|v</em>{\rm bulk}|$, their ratio $R_v \equiv |v_{\rm bulk}|/σ_v$, and non-thermal pressure proxies. Disturbed non-cool-core systems are not simply higher-dispersion counterparts of relaxed cool-core regions. Instead, differences among cool-core centers, cool-core outer regions, and non-cool-core systems are driven mainly by coherent line-of-sight motion relative to unresolved line broadening: $R_v$ tends to remain below unity in cool-core regions but often exceeds unity in non-cool-core systems, with the mean $R_v$ rising from $0.45$ in cool-core centers to $1.6$ in non-cool-core systems. These diagnostics help separate local central line broadening, likely associated with AGN feedback in some cool cores, from larger-scale coherent motions associated with sloshing, mergers, and halo assembly. Comparison with forward-modeled TNG-Cluster predictions suggests that many cool-core measurements occupy the lower part of the predicted non-thermal pressure range, consistent with small hydrostatic-mass corrections in relaxed systems and larger corrections in disturbed ones. XRISM is thus beginning to resolve the projected kinetic structure of the ICM across cluster environments, rather than tracing a single sequence of increasing turbulence.

• The paper shows that disturbed non-cool-core clusters have substantially stronger coherent bulk motions, with mean |v_bulk| rising to 319 km s⁻¹ and R_v reaching 1.58, rather than simply having more turbulence.
• The study homogenizes 45 measurements across 19 nearby clusters using the effective line-of-sight scale, finding subsonic 3D Mach numbers of 0.20–0.28 while separating unresolved broadening from coherent gas flows.
• The results indicate that relaxed cool cores generally have only a few percent non-thermal pressure, whereas some disturbed clusters exceed 10%, highlighting potential hydrostatic mass bias and tension with TNG-Cluster predictions.

Motivation and diagnostic framework

Gas motions in the intracluster medium (ICM) carry direct information about halo assembly, sloshing, and AGN feedback, and they contribute non-thermal pressure that biases hydrostatic mass estimates used in cluster cosmology. High-resolution X-ray spectroscopy with XRISM/Resolve now permits direct measurement of ICM velocities through Doppler shifts and line broadening of the Fe–K complex. This Letter by Ota et al. compiles 45 XRISM/Resolve measurements across 19 nearby galaxy clusters into a common diagnostic framework, comparing the line-of-sight velocity dispersion $\sigma_v$, the bulk velocity amplitude $|v_{\rm bulk}|$, their ratio $R_v \equiv |v_{\rm bulk}|/\sigma_v$, and non-thermal pressure proxies as a function of an emission-weighted effective line-of-sight scale $\ell_{\rm eff}$.

The central methodological contribution is the homogenization of heterogeneous measurements onto a common aperture scale. Following Zhuravleva et al., each region is assigned $\ell_{\rm eff}$: the half-length of the sky-plane-centered line-of-sight interval containing 50% of the emission measure, computed from single- or double-$\beta$ density models (largely from ACCEPT and HIFLUGCS profiles) integrated to $R_{200}$. The sample spans nearly two orders of magnitude in $\ell_{\rm eff}$, from a few kpc in central cool cores to several hundred kpc in disturbed systems. The authors are explicit that both diagnostics are emissivity-weighted projected quantities affected by projection, PSF mixing, and aperture averaging; $\sigma_v$ is interpreted as unresolved velocity structure within the aperture rather than turbulence alone.

The velocity partition result

The paper's principal empirical finding is that disturbed non-cool-core (NCC) systems are not simply higher-dispersion versions of relaxed cool cores. Binned by dynamical state (CC center, CC outer, NCC), the mean $|v_{\rm bulk}|$ rises from 56 to 319 km s$|v_{\rm bulk}|$0 while mean $|v_{\rm bulk}|$1 rises only from 135 to 232 km s$|v_{\rm bulk}|$2; the turbulent Mach number $|v_{\rm bulk}|$3 remains subsonic and nearly constant ($|v_{\rm bulk}|$4–$|v_{\rm bulk}|$5) because NCC systems are hotter. The mean velocity ratio climbs from $|v_{\rm bulk}|$6 in cool-core centers to $|v_{\rm bulk}|$7 in NCC systems, with most CC-center regions below unity and many NCC regions above it. The transition between dynamical states is therefore driven mainly by the growing coherent line-of-sight component relative to unresolved broadening, not by a uniform increase in Mach-scaled turbulence strength.

Several robustness checks support this interpretation. Re-referencing the strongest mergers (Coma, A1914, A2034) to plausible BCG frames shifts the NCC mean only within $|v_{\rm bulk}|$8–$|v_{\rm bulk}|$9; excluding them still yields $R_v \equiv |v_{\rm bulk}|/\sigma_v$0, well above the CC-center value. Cluster-level averaging rather than region-level statistics recovers the same qualitative trends. However, the authors candidly note that aperture scale and dynamical state are partly coupled — mean $R_v \equiv |v_{\rm bulk}|/\sigma_v$1 rises from 34 kpc (CC center) through 117 kpc (CC outer) to 133 kpc (NCC) — and that Spearman correlations of $R_v \equiv |v_{\rm bulk}|/\sigma_v$2 and $R_v \equiv |v_{\rm bulk}|/\sigma_v$3 with $R_v \equiv |v_{\rm bulk}|/\sigma_v$4 weaken under cluster resampling, so the scale trends are treated as suggestive rather than statistically robust. Notably, CC outer and NCC regions have comparable mean $R_v \equiv |v_{\rm bulk}|/\sigma_v$5 yet very different $R_v \equiv |v_{\rm bulk}|/\sigma_v$6 and $R_v \equiv |v_{\rm bulk}|/\sigma_v$7, indicating that dynamical state, not scale alone, controls the velocity partition.

Non-thermal pressure and hydrostatic mass bias

Two pressure proxies quantify the kinetic importance of gas motions: $R_v \equiv |v_{\rm bulk}|/\sigma_v$8, computed from $R_v \equiv |v_{\rm bulk}|/\sigma_v$9 alone assuming isotropic random motion, and $\ell_{\rm eff}$0, which adds the coherent bulk velocity via an effective three-dimensional Mach number. In relaxed cool-core regions both proxies are typically a few per cent, implying small hydrostatic mass corrections on the sampled apertures if the non-thermal fraction varies slowly with radius. Some NCC regions reach $\ell_{\rm eff}$1 — for example Coma South ($\ell_{\rm eff}$2), A2034 ($\ell_{\rm eff}$3), and Perseus E+NE ($\ell_{\rm eff}$4) — signaling potentially appreciable departures from hydrostatic equilibrium. The authors caution that these are projected per-aperture estimates, not radial deprojections, and that the actual mass bias depends on radial gradients, anisotropy, and whether coherent motions act as pressure support.

Comparison with forward-modeled TNG-Cluster predictions (spectroscopically weighted synthetic Resolve observations along three axes) shows the XRISM points occupying the same broad parameter space but preferentially populating the lower part of the predicted non-thermal pressure range, in both CC centers and several CC outer pointings. If confirmed, this suggests simulations retain too much unresolved outer-core motion and possibly too much coherent core motion — a concrete tension worth further investigation. NCC pointings instead extend toward the simulated disturbed branch, consistent with expectations for late-forming, actively assembling halos.

Physical drivers: assembly versus feedback

The CC-center-to-NCC progression aligns with a relaxation picture: dynamically old cores have decayed merger-driven motions, while NCC systems trace recent or ongoing assembly. AGN feedback acts as a more local driver in cool-core centers, elevating $\ell_{\rm eff}$5 in systems such as Virgo/M87, Cygnus A, and H1821+643. An instructive contrast emerges between feedback channels: quasar-host systems remain below unity in $\ell_{\rm eff}$6, suggesting radiative feedback alone does not produce the high-$\ell_{\rm eff}$7 tail, which is dominated by sloshing, mergers, and large-scale coherent flows. The large scatter among CC outer regions reflects sensitivity to viewing direction and aperture placement — low $\ell_{\rm eff}$8 does not exclude coherent sloshing, since motions coherent across the Resolve aperture appear as centroid shifts rather than line broadening.

Limitations and open questions

The paper concedes several limitations plainly. The CC/NCC classification is phenomenological rather than a strict merger taxonomy; only one weak-cool-core system (A3571) is present, forcing WCC to be merged with CC. Reference-frame choices for mergers without a well-defined BCG introduce systematic uncertainty in $\ell_{\rm eff}$9, bounded by the re-referencing test described above. Formal correlation significances are likely optimistic given multiple regions per cluster. The key open question the authors pose is whether larger, uniformly selected samples with matched-$\ell_{\rm eff}$0 coverage and homogeneous AGN-power estimates will establish the $\ell_{\rm eff}$1–$\ell_{\rm eff}$2 behavior as a robust physical organization of ICM kinematic states, and whether the apparent quiet-end mismatch with TNG-Cluster persists.

Conclusion

This compilation demonstrates that XRISM/Resolve is resolving the projected kinetic structure of the ICM across cluster environments rather than tracing a single sequence of increasing turbulence. The velocity ratio $\ell_{\rm eff}$3, combined with non-thermal pressure proxies, provides a compact diagnostic separating local central line broadening associated with AGN feedback from larger-scale coherent motions tied to sloshing, mergers, and halo assembly. The small non-thermal fractions measured in relaxed cool cores support modest hydrostatic mass corrections there, while the high-$\ell_{\rm eff}$4, high-$\ell_{\rm eff}$5 disturbed systems identify where mass calibration and turbulence-only interpretations of line widths require greater caution.