XRISM Observation of the Ophiuchus Galaxy Cluster: Quiescent Velocity Structure in the Dynamically Disturbed Core

TL;DR

XRISM's high-resolution X-ray observations of the Ophiuchus galaxy cluster reveal a surprisingly quiescent core with low turbulence and near-rest gas, challenging previous notions of its dynamical state and indicating a potential transition in core activity.

Contribution

This study provides the first high-resolution X-ray spectroscopic measurements showing low gas velocity dispersions in a dynamically disturbed cluster core, revealing minimal turbulence and near-static gas motion.

Findings

Inner region gas velocity dispersion ~115 km/s

Outer region gas velocity dispersion ~186 km/s

Nonthermal pressure fractions below 3%

Abstract

We present the high-resolution X-rayspectroscopic observations of the Ophiuchus galaxy cluster core using the XRISM satellite. Despite previous observations revealing multiple cold fronts and dynamical disturbances in the cluster core, our XRISM observations show low gas velocity dispersions of sigma_v = 115 +/- 7 km s^-1 in the inner region (~< 25 kpc) and sigma_v = 186 +/- 9 km s^-1 in the outer region (~ 25-50 kpc). The gas temperatures are kT = 5.8 +/- 0.2 keV and 8.4 +/- 0.2 keV for the inner and outer regions, respectively, with metal abundances of Z = 0.75 +/- 0.03 Z_sun (inner) and 0.44 +/- 0.02 Z_sun (outer). The measured velocity dispersions correspond to nonthermal pressure fractions of only 1.4 +/- 0.2% (inner) and 2.5 +/- 0.2% (outer), indicating highly subsonic turbulence. Our analysis of the bulk gas motion indicates that the gas in the inner region is nearly at rest relative to the central galaxy (|v_bulk| = 8 +/- 7 km s^-1), while the outer region exhibits a moderate motion of |v_bulk| = 104 +/- 7 km s^-1. Assuming the velocity dispersion arises from turbulent motions, the turbulent heating rate is ~ 40\% of the radiative cooling rate, although there is some uncertainty. This suggests that the heating and cooling of the gas are not currently balanced. The activity of the central active galactic nucleus (AGN) has apparently weakened. The sloshing motion that created the cold fronts may now be approaching a turning point at which the velocity is minimum. Alternatively, the central galaxy and the associated hot gas could be moving nearly parallel to the plane of the sky.