Three‐Dimensional Simulations of Viscous Dissipation in the Intracluster Medium

We present three-dimensional simulations of viscous dissipation of AGNinduced gas motions and waves in clusters of galaxies. These simulations aremotivated by recent detections of ripples in the Perseus and Virgo clusters.Although the sound waves generated by buoyant bubbles decay with distance fromthe cluster center, we show that these waves can contribute substantially tooffsetting the radiative cooling at distances significantly exceeding thebubble size. The energy flux of the waves declines more steeply with radiusthan the inverse-square law predicted by energy conservation, implying thatdissipation plays an important role in tapping the wave energy. We show thatsuch dispersing sound waves/weak shocks are detectable as ripples onunsharp-masked X-ray cluster maps, and point out that the interfaces betweenthe intracluster medium and old bubbles are also clearly detectable inunsharp-masked X-ray maps. This opens up the possibility of detecting fossilbubbles that are difficult to detect in radio emission. This mode of heating isconsistent with other observational constraints, such as the presence of coolrims around the bubbles and the absence of strong shocks. Thus, the mechanismoffers a way of heating clusters in a spatially distributed and gentle fashion.We also discuss the energy transfer between the central AGN and the surroundingmedium. In our numerical experiments, we find that roughly 65 per cent of theenergy injected by the AGN is transferred to the intracluster medium andapproximately 25 percent of the injected energy is dissipated by viscouseffects and contributes to heating of the gas. The overall transfer of heatfrom the AGN to the gas is comparable to the radiative cooling losses. Thesimulations were performed with the FLASH adaptive mesh refinement code.Comment: accepted for Ap