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

TL;DR

XRISM observations of Abell 2029 show very low non-thermal pressure and turbulence in its core, indicating it is an exceptionally relaxed galaxy cluster with minimal recent disturbance.

Contribution

First precise measurement of ICM velocity dispersion and non-thermal pressure in Abell 2029 using XRISM Resolve, confirming its relaxed state with minimal turbulence.

Findings

Line-of-sight bulk velocity consistent with rest frame

ICM velocity dispersion of 169 +/- 10 km/s

Non-thermal pressure fraction of 2.6% +/- 0.3%

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.