Cluster heating by viscous dissipation of sound waves

We simulate the effects of viscous dissipation of waves that are generated by active galactic nucleus (AGN) activity in clusters of galaxies. We demonstrate that the amount of viscous heating associated with the dissipation of these waves can offset radiative cooling rates in cooling flow clusters of galaxies. This heating mechanism leads to spatially distributed and approximately symmetrical dissipation. The heating waves reach a given distance from the cluster center on a timescale shorter than the cooling time. This means that this heating mechanism has the potential to quench cooling flows in a quasi-stable fashion. Moreover, the heating is gentle, as no strong shocks are present in the simulations. We first investigated whether a single continuous episode of AGN activity can lead to adequate dissipation to balance cooling rates. These simulations demonstrated that whereas secondary waves generated by the interaction of the rising bubble with the intracluster medium are clearly present, viscous heating associated with the dissipation of these waves is insufficient to balance radiative cooling. It is only when the central source is intermittent that the viscous dissipation of waves associated with subsequent episodes of activity can offset cooling. This suggests that the ripples observed in the Perseus Cluster can be interpreted as being due to the AGN duty cycle; i.e., they trace AGN activity history. The simulations were performed using the adaptive mesh refinement code FLASH in two dimensions.