Acoustic waves and turbulence as energy carriers in a viscous intracluster medium

Many recent works on the observed galaxy clusters in the X-rays highlight broadly two classes of exclusive energy carriers — sound waves and turbulence. Weak or moderately strong shocks generated by AGN feedback are expected to decay into fast outgoing sound waves. Turbulence can be fed by non-linear g-modes but g-modes are also expected to be tightly confined to the cluster core in linear regime. Consequently we expect the fastest g-modes to not contribute to the energetics in the outskirts of the core. But low density bubbles generated by AGN feedback can buoyantly rise and seed vorticity in the outskirts. It is not straight-forward to argue in favour of only one of these two fundamental energy carriers — compressible (shocks/sound waves) and incompressible (g-modes/mixing/turbulence) modes — to dominantly contribute to the prevention of cooling-flows in a cluster. In order to understand this dichotomy, we use idealized three-dimensional hydrodynamic simulations of intracluster medium (ICM) with gentle to moderate feedback energy injection (the gentle and strong feedbacks being categorised by slow and fast injection rates). While instantaneous bursts of energy injection have been shown to produce strong shocks and evacuation in simulated clusters to easily prevent cooling, X-ray/radio observations of Perseus cluster motivates a gentler flow of energy across the ICM core. We take a cue from such observations and model a gentle to moderately strong regime of feedback in which energy transfer may occur by the aforementioned fundamental energy carriers. We find interesting gentle-to-moderate feedback regimes in which either sound waves contribute dominantly to viscous dissipation or turbulence does. However, we also find that sound wave is the most feasible mode of transport to a distance of >100 kpc. Broadly, we conclude that both the energy carriers may have complementary roles in the prevention of cooling-flows.