Efficient Production of Sound Waves by AGN Jets in the Intracluster Medium

Abstract: We investigate the interaction between active galactic nuclei (AGN) jets and the intracluster medium (ICM) of galaxy clusters. Specifically, we study the efficiency with which jets can drive sound waves into the ICM. Previous works focused on this issue model the jet-ICM interaction as a spherically symmetric explosion, finding that <~ 12.5% of the blast energy is converted into sound waves, even for instantaneous energy injection. We develop a method for measuring sound wave energy in hydrodynamic simulations and measure the efficiency of sound wave driving by supersonic jets in a model ICM. Our axisymmetric fiducial simulations convert >~ 25% of the jet energy into strong, long-wavelength sound waves which can propagate to large distances. Vigorous instabilities driven by the jet-ICM interaction generate small-scale sound waves which constructively interfere, forming powerful large-scale waves. By scanning a parameter space of opening angles, velocities, and densities, we study how our results depend on jet properties. High velocity, wide angle jets produce sound waves most efficiently, yet the acoustic efficiency never exceeds 1/3 of the jet energy -- an indication that equipartition may limit the nonlinear energy conversion process. Our work argues that sound waves may comprise a significant fraction of the energy budget in cluster AGN feedback and underscores the importance of properly treating compressive wave dissipation in the weakly collisional, magnetized ICM.