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

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