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X-ray observations of galaxy clusters have detected numerous X-ray cavities,evolved from the interaction of AGN jets with the intracluster medium (ICM) andproviding compelling evidence for the importance of jet-mode AGN feedback. Herewe argue for the physical importance of the cavity shape, which we characterizewith two geometric parameters: radial elongation $\tau$ and top wideness $b$.We study the cavity shape with 16 hydrodynamic jet simulations in tworepresentative clusters, and find that the shapes of young cavities are mainlydetermined by various jet properties. Our simulations successfully reproducetwo observed types of young cavities elongated along either the jet ($\tau>1$;type-II) or perpendicular ($\tau\leq1$; type-I) direction. Bottom-wide type-Icavities are produced by very light internally-subsonic jets, while top-widetype-II cavities are produced by heavier, internally-supersonic jets, which mayalso produce center-wide cavities with $\tau\sim 1$ if the jets are onlyslightly supersonic. Bottom-wide type-II cavities can be produced by very lightjets with very long durations and cylindrical cavities are produced by verylight internally-supersonic jets. While not appreciably affecting the shapes ofyoung cavities, viscosity significantly affects the long-term cavity evolution,suppressing both interface instabilities and the formation of torus-likemorphology. We encourage observers to study the shapes of young and old X-raycavities separately, the former probing the properties of AGN jets and thelatter potentially probing the ICM viscosity level.

The Shape of X-ray Cavities in Galaxy Clusters: Probing Jet Properties and Viscosity