Dynamics and stability of magnetized AGN-blown bubbles in clusters of galaxies

Abstract

We perform MHD simulations of AGN-blown bubbles in the Intercluster Medium (ICM) containing large-scale coherent magnetic fields. We assume that bubbles, created by the intermittent jets from Active Galactic Nuclei, quickly relax to the Woltjer-Taylor spheromak-like state, with internal plasma beta-parameter $\sim 1$. We demonstrate that such bubbles rising through hydrostatically-stratified atmosphere are magnetically stabilized against fluid interface instabilities, remaining coherent for a long time. Typical velocity is $ v /c_s \sim \sqrt{R/H} \leq 1 $ ($c_s$ is sound speed, $R$ is the bubble size, $H$ is the scale height). Current-driven instabilities (internal kinks) lead to bubble's tilting, but develop on long time scales, and remain unimportant, leading to minor modifications of the internal structure. Our results explain apparent long-term stability of ICM cavities. Subsonically rising stable bubbles dissipate in their wake approximately the energy initially injected by the jet, and may efficiently reheat the clusters cores in a ``gentle'' way.

Figures (8)

• Figure 1: Cartoon of the mode. Magnetized bubble, modeled as pressure-confined, mildly magnetized $\beta_{0,in} \leq 1$ spheromaks. Initially, the bubbles has radius $R_0 \leq H$, The bubble is submerged into unmagnetized, gravitationally stratified ICM. Two cases of magnetic field orientation within the bubble, horizontal and vertical, are investigated.
• Figure 2: Pressure-confined magnetic bubble. Left panel: 3-D rendering of the twisted magnetic fields embedded in the bubble, resembling a "ball of yarn". Right panel: 2-D slice of initial density variation $\Delta \rho/\rho$ and magnetic field streamlines. Note that the B field is axisymmetric, and any asymmetry in the streamlines are merely a plotting artifact. In the set-up the pressure is matched at the top of the bubble and is mildly discontinuous elsewhere on the surface. Due to the external pressure gradient the bubble will experience a buoyant force propelling it upwards.
• Figure 3: Renderings of $\beta=10$ isosurfaces of a buble of initial radius $R=.96H$. Left to right: view from the outside, zooming in from the inside to the bubble's core (note the kink-twisted structure), and viewing bottom-up. From the internal view, we can see the tight twisting and the resulting asymmetry due to kink instability. The bubble also acquiers sheared toroidal velocity 2011SoPh..270..537L. Here color is just the z coordinate.
• Figure 4: Bubble dynamics depending on the initial size, vertically aligned case, no outside magnetic field. Plotted are midplane slices of $\log(\beta)$ for three cases of varying initial radius $R=0.2, \, 0.33,\, 0.96$, at times $0,3,6,9$$\times H/c_s$.
• Figure 5: Location and rising speed for vertically oriented bubbles of various initial radius (scales as $(5.76/\alpha) *H$). After an initial equilibration period, the bubbles approach a constant rising velocity, with the larger bubbles rising faster. The largest bubble ($R=.96$) encounters a plunger like effect that inhibits its rising as it runs against the side walls of the domain past timestep 600. We include an intermediate size run ($R=.64$) to fill out the parameter space.