Draping of Cluster Magnetic Fields over Bullets and Bubbles—Morphology and Dynamic Effects

Abstract

High-resolution X-ray observations have revealed cavities and ''cold fronts'' with sharp edges in temperature and density within galaxy clusters. Their presence poses a puzzle, since these features are not expected to be hydrodynamically stable or to remain sharp in the presence of diffusion. However, a moving core or bubble in even a very weakly magnetized plasma necessarily sweeps up enough magnetic field to build up a dynamically important sheath; the layer's strength is set by a competition between ''plowing up'' and slipping around of field lines, and depends primarily on the ram pressure seen by the moving object. In this inherently three-dimensional problem, our analytic arguments and numerical experiments show that this layer modifies the dynamics of a plunging core, greatly modifying the hydrodynamic instabilities and mixing, changing the geometry of stripped material, and slowing the core through magnetic tension. We derive an expression for the maximum magnetic field strength and thickness of the layer, as well as for the opening angle of the magnetic wake. The morphology of the magnetic draping layer implies the suppression of thermal conduction across the layer, thus conserving strong temperature gradients. The intermittent amplification of the magnetic field as well as the injection of magnetohydrodynamic turbulence in the wake of the core is identified to be due to vorticity generation within the magnetic draping layer. These results have important consequences for understanding the complex gas-dynamical processes of the intracluster medium and apply quite generally to motions through other magnetized environments, e.g., the interstellar medium.

Interplay Among Cooling, Agn Feedback, and Anisotropic Conduction in the Cool Cores of Galaxy Clusters

“…While currently treated as hot gas in the simulations, the jets are likely composed of relativistic particles, for which the Spitzer conductivity does not apply. As for the shocked ICM, although the conductivity is high by face values, the actual heat flux from the shocked to ambient gas is suppressed because magnetic field is aligned with the shock surface due to magnetic draping (Lyutikov 2006;Dursi & Pfrommer 2008). Indeed, we find that varying the ceiling does not significantly affect our main conclusions.…”

“…Magnetic fields may increase the structural integrity of jet-blown cavities, especially if enhanced by the action of magnetic draping as the cavity buoyantly rises (Dursi & Pfrommer 2008). This may, in turn, promote the driving of turbulence in and subsequent heating of the ICM (this possibility was speculated on in Reynolds et al 2015).…”

The bow shock, cold fronts and disintegrating cool core in the merging galaxy group RX J0751.3+5012

“…Vikhlinin et al 2001b;Dursi & Pfrommer 2008). Kelvin-Helmholz instabilities can be suppressed by this magnetic layer that is aligned with the interface and the flow direction.…”

Applications for edge detection techniques usingChandraandXMM–Newtondata: galaxy clusters and beyond

“…Lyutikov 2006, Asai et al 2007, Dursi & Pfrommer 2008. This would severely inhibit transport processes and prevent cold fronts from disintegrating through Kelvin-Helmholtz instabilities (KHIs), which would otherwise be expected to develop on relatively short timescales (Chandrasekhar 1961).…”