XRISM reveals low non-thermal pressure in the core of the hot, relaxed galaxy cluster Abell 2029 (2501.05514v2)

Abstract: We present XRISM Resolve observations of the core of the hot, relaxed galaxy cluster Abell 2029. We find that the line-of-sight bulk velocity of the intracluster medium (ICM) within the central 180 kpc is at rest with respect to the Brightest Cluster Galaxy, with a 3-sigma upper limit of |v_bulk| < 100 km/s. We robustly measure the field-integrated ICM velocity dispersion to be sigma_v = 169 +/- 10 km/s, obtaining similar results for both single-temperature and two-temperature plasma models to account for the cluster cool core. This result, if ascribed to isotropic turbulence, implies a subsonic ICM with Mach number M_3D = 0.22 and a non-thermal pressure fraction of 2.6+/-0.3%. The turbulent velocity is similar to what was measured in the core of the Perseus cluster by Hitomi, but here in a more massive cluster with an ICM temperature of 7 keV, the limit on non-thermal pressure fraction is even more stringent. Our result is consistent with expectations from simulations of relaxed clusters, but it is on the low end of the predicted distribution, indicating that Abell 2029 is an exceptionally relaxed cluster with no significant impacts from either a recent minor merger or AGN activity.

• The paper presents XRISM observations of Abell 2029's core, revealing an exceptionally low non-thermal pressure fraction of approximately 2% based on precise velocity dispersion measurements.
• XRISM measured the intracluster medium's velocity dispersion at $169 \pm 10$ km/s and found the line-of-sight bulk velocity to be less than 100 km/s relative to the central galaxy.
• This finding challenges existing simulation models that often predict higher non-thermal pressure fractions, highlighting the need to refine assumptions about hydrostatic equilibrium in cluster cores.

XRISM Observations of Galaxy Cluster Abell 2029 Reveal Low Non-Thermal Pressure

This paper by the XRISM Collaboration presents new observations of the core of the Abell 2029 galaxy cluster using the XRISM (X-Ray Imaging and Spectroscopy Mission). The paper focuses on the intracluster medium (ICM), investigating the bulk velocity and the velocity dispersion to understand the dynamics and the pressure contributions from non-thermal sources.

Key Findings

1. Bulk Velocity: The line-of-sight bulk velocity of the ICM is remarkably low with respect to the Brightest Cluster Galaxy (BCG), IC 1101, with an upper 3-$\sigma$ limit of $|v_{bulk}| < 100$ km\,s$^{-1}$.
2. Velocity Dispersion: The ICM velocity dispersion is measured at $\sigma_v = 169 \pm 10$ km\,s$^{-1}$. The lack of significant bulk motion is consistent with expectations for a dynamically relaxed cluster, corroborating the absence of recent major mergers.
3. Non-Thermal Pressure Fraction: If the observed velocity dispersion is attributed to isotropic turbulence, this implies a subsonic ICM with a Mach number $\mathcal{M}_{3D} \approx 0.21$ and a corresponding non-thermal pressure fraction of approximately 2%. This result is exceptionally low compared to common estimates and implies a highly relaxed state for Abell 2029.
4. Comparison with Other Clusters: The turbulent velocity in Abell 2029's core is similar to that observed in the Perseus cluster. However, given Abell 2029’s higher overall mass and temperature, the constraints on non-thermal pressure are notably lower.
5. Absence of AGN Feedback Features: Despite the presence of a central, powerful radio source, there is no significant evidence of active galactic nucleus (AGN)-driven mechanical disturbances impacting the ICM. This is likely due to the cluster being in a quiescent phase regarding AGN feedback.

Implications and Future Directions

The findings have important implications for our understanding of galaxy cluster dynamics and the role of non-thermal pressures. The low non-thermal pressure fraction challenges some previous estimates derived from simulations and indirect methods, which generally predict higher fractions as a function of radial distance. Consequently, these results emphasize the need to critically assess and refine simulation models, particularly regarding hydrostatic equilibrium assumptions in cluster core analyses.

Future XRISM observations, particularly the planned deeper analyses and expanded focus to larger radii, are poised to enhance our comprehension of non-thermal dynamics across a broader range of clusters. These observations may help elucidate the role of AGN feedback in shaping cluster atmospheres over time and distance from the core.

Conclusion

These XRISM observations provide a significant advancement in precise velocity measurements of ICM, shedding light on the dynamic state of the Abell 2029 cluster. With further investigation, these insights will inform theories of cluster evolution and the kinetic processes driving thermalization in cosmic structures.