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 agrees much better with a model where the
cavities are magnetically dominated and inflated by a current-dominated
magneto-hydrodynamic jet model, recently developed by Li et al. (2006) and
Nakamura et al. (2006). 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 1sigma, whereas the other models can be excluded at >5sigma
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 active galactic nucleus feedback.Comment: 21 pages, 12 figures, emulateapj, accepted for publication in ApJ,
responded to referee's comments and added a new model, conclusions unchange
