The splash beneath the largest radio bubble in a cluster core

TL;DR

This study uses XRISM observations to analyze turbulence and gas motions in the wake of a large radio bubble in the Ophiuchus cluster, finding modest turbulence insufficient to offset cooling.

Contribution

First direct measurement of turbulence and bulk motions in the wake of a giant radio bubble in a galaxy cluster core.

Findings

Detected a velocity shift of -80 km/s in the bubble's wake.

Velocity dispersion increases from 135 km/s to 210 km/s in the wake.

Turbulent energy is only 1% of the thermal energy, insufficient to prevent cooling.

Abstract

We present a 100 ks XRISM Resolve observation of the Ophiuchus cluster that measures turbulence and bulk motion in the wake of the largest radio bubble on the sky. We detect a significant velocity shift of $-80\pm20$ km/s from the cluster centre to the bubble's wake and a clear increase in velocity dispersion from $135\pm10$ km/s to $210\pm20$ km/s. The measured bulk velocity in the wake is low and suggests that the bubble's trajectory is inclined with respect to the line of sight. If we subdivide the bubble's wake, fitting spectra simultaneously with cross-region responses, we find that the velocity shift and dispersion increase are primarily detected in the very centre of the wake. This is consistent with the expected updraft, or `splash', found beneath buoyantly rising radio bubbles. In the cluster's cool core, the turbulent kinetic energy is only 1% of the thermal energy radiated over a cooling timescale of 7 Gyr, and even falls short, by a factor of 5, of the thermal energy radiated over the bubble's rise time. Whilst turbulent energy generated in the large wake region may provide additional heating, this propagates too slowly to prevent rapid cooling across the core. The turbulent-dissipation heating rate is a factor of ~3 below the cooling luminosity. Despite the vast power of the giant radio bubble in the Ophiuchus cluster, the gas motions in the wake are remarkably modest and turbulent-dissipation appears unable to prevent rapid cooling.