Three-Dimensional Magnetohydrodynamic Simulations of Buoyant Bubbles in Galaxy Clusters

Abstract

We report results of 3D MHD simulations of the dynamics of buoyant bubbles in magnetized galaxy cluster media. The simulations are three dimensional extensions of two dimensional calculations reported by Jones & De Young (2005). Initially spherical bubbles and briefly inflated spherical bubbles all with radii a few times smaller than the intracluster medium (ICM) scale height were followed as they rose through several ICM scale heights. Such bubbles quickly evolve into a toroidal form that, in the absence of magnetic influences, is stable against fragmentation in our simulations. This ring formation results from (commonly used) initial conditions that cause ICM material below the bubbles to drive upwards through the bubble, creating a vortex ring; that is, hydrostatic bubbles develop into "smoke rings", if they are initially not very much smaller or very much larger than the ICM scale height.Even modest ICM magnetic fields with β = P gas /P mag 10 3 can influence the dynamics of the bubbles, provided the fields are not tangled on scales comparable to or smaller than the size of the bubbles. Quasi-uniform, horizontal fields with initial β ∼ 10 2 bifurcated our bubbles before they rose more than about a scale height of the ICM, and substantially weaker fields produced clear distortions. These behaviors resulted from stretching and amplification of ICM fields trapped in irregularities along the top surface of the young bubbles. On the other hand, tangled magnetic fields with similar, modest strengths are generally less easily amplified by the bubble motions and are thus less influential in bubble evolution. Inclusion of a comparably strong, tangled magnetic field inside the initial bubbles had little effect on our bubble evolution, since those fields were quickly diminished through expansion of the bubble and reconnection of the initial field.

Simulations of the dynamics of magnetized jets and cosmic rays in galaxy clusters

“…This draping layer inhibits any particle transport across the bubble surface such as CR diffusion (Ruszkowski et al 2008), heat conduction by thermal electrons and momentum transport or viscosity by thermal protons, which stabilises observed sharp temperature and density transitions in the ICM (i.e., cold fronts) against disruptions (Vikhlinin et al 2001;Lyutikov 2006;Asai et al 2007). Previous simulations acknowledge the stabilising effect of magnetic fields (Jones & De Young 2005;O'Neill et al 2009;Bambic et al 2018). Here, for the first time, we present results for the case of self-consistently inflated bubbles in a realistic turbulent magnetised environment.…”

Connecting turbulent velocities and magnetic fields in galaxy cluster simulations with active galactic nuclei jets

“…At 50 Myr, lobes are inflated, which rise buoyantly as bubbles in the cluster. Their powerful wake causes magnetic field amplification and drags up gas from the center (Jones & De Young 2005;O'Neill et al 2009). The later process is visible as filamentous structures of enhanced column densities that extend from the centre of the cluster to the bubbles.…”

Using collisions of AGN outflows with ICM shocks as dynamical probes

“…The cocoon plasma will generally be of substantially lower density than the surrounding ICM, with the two media nominally separated by a contact discontinuity. Because it is also a "slip surface", that boundary is likely to be unstable to Kelvin-Helmholtz instabilities (KHI), so that some degree of mixing will take place (unless, for instance, magnetic fields in or around the cocoon are strong enough and coherent enough to stabilize the contact discontinuity 34,35 ). Figure 4 shows volume renderings of cocoons formed from two simulated steady AGN jet pairs.…”