2025PASJ...77S.270F

We present high-resolution X-ray spectroscopic observations of the Ophiuchus galaxy cluster core using the X-Ray Imaging and Spectroscopy Mission (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 $\sigma _{\rm v} = 115\pm 7$ km s$^{-1}$ in the inner region ($\lesssim$25 kpc) and $\sigma _{\rm v} = 186\pm 9$ km s$^{-1}$ in the outer region ($\sim$25-50 kpc). The gas temperatures are $kT = 5.8\pm 0.2$ and $8.4\pm 0.2$ keV for the inner and outer regions, respectively, with metal abundances of $Z = 0.75\pm 0.03$ Z$_{\odot }$ (inner) and $0.44\pm 0.02$ Z$_{\odot }$ (outer). The measured velocity dispersions correspond to non-thermal pressure fractions of only $1.4\pm 0.2$% (inner) and $2.5\pm 0.2$% (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 ($|v_{\rm bulk}|=8\pm 7$ km s$^{-1}$), while the outer region exhibits a moderate motion of $|v_{\rm bulk}|=104\pm 7$ km s$^{-1}$. Assuming the velocity dispersion arises from turbulent motions, the turbulent heating rate is $\sim$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 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.