2008ApJ...687..173D

We present results from an extensive survey of 64 cavities in the X-ray halos of clusters, groups, and normal elliptical galaxies. We show that the evolution of the size of the cavities as they rise in the X-ray atmosphere is inconsistent with the standard model of adiabatic expansion of purely hydrodynamic models. We also note that the majority of the observed bubbles should have already been shredded apart by Rayleigh-Taylor and Richtmyer-Meshkov instabilities if they were of purely hydrodynamic nature. Instead, we find that the data agree much better with a model where the cavities are magnetically dominated and inflated by a current-dominated MHD jet model, recently developed by Li and coworkers and Nakamura and coworkers. We conduct complex Monte Carlo simulations of the cavity detection process including incompleteness effects to reproduce the cavity sample's characteristics. We find that the current-dominated model agrees within 1 σ, whereas the other models can be excluded at >5 σ confidence. To bring hydrodynamic models into better agreement, cavities would have to be continuously inflated. However, these assessments are dependent on our correct understanding of the detectability of cavities in X-ray atmospheres and will await confirmation when automated cavity detection tools become available in the future. Our results have considerable impact on the energy budget associated with AGN feedback.