Three‐dimensional Magnetohydrodynamic Simulations of Cold Fronts in Magnetically Turbulent ICM

Abstract

Steep gradients of temperature and density, called cold fronts, are observed by Chandra in a leading edge of subclusters moving through the intracluster medium (ICM). The presence of cold fronts indicates that thermal conduction across the front is suppressed by magnetic fields. We carried out three-dimensional magnetohydrodynamic (MHD) simulations including anisotropic thermal conduction of a subcluster moving through a magnetically turbulent ICM. We found that turbulent magnetic fields are stretched and amplified by shear flows along the interface between the subcluster and the ambient ICM. Since magnetic fields reduce the efficiency of thermal conduction across the front, the cold front survives for at least 1 Gyr. We also found that a moving subcluster works as an amplifier of magnetic fields. Numerical results indicate that stretched turbulent magnetic fields accumulate behind the subcluster and are further amplified by vortex motions. The moving subcluster creates a long tail of ordered magnetic fields, in which the magnetic field strength attains a value of ¼ P gas /P mag P 10.

Magnetohydrodynamic Simulations of the Formation of Cold Fronts in Clusters of Galaxies: Effects of Anisotropic Viscosity

“…We note that implicit anisotropic heat conduction is non-monotonic (Sharma & Hammett 2007). This non-monotonicity did not crash the simulations based on the modified Lax-Wendroff scheme applied to solar flares (e.g., Yokoyama & Shibata 2001) and cold fronts in cluster of galaxies (Asai et al 2004(Asai et al , 2005(Asai et al , 2007. We confirmed that our HLLD code combined with the implicit solver for the heat conduction reproduces the results of three-dimensional MHD simulations with heat conduction reported by Asai et al (2005).…”

“…Without it, the sharp temperature discontinuity quickly disappears because the subcluster plasma is heated by the hot ambient plasma, as mentioned by Ettori & Fabian (2000). Asai et al (2004Asai et al ( , 2005Asai et al ( , 2007 also confirmed the suggestion by Vikhlinin et al (2001aVikhlinin et al ( , 2001b) that the magnetic fields stretched and strengthened along the front suppress the growth of the Kelvin-Helmholtz (K-H) instability at the interface between the subcluster plasma and the ambient plasma.…”

“…Asai et al (2004Asai et al ( , 2005Asai et al ( , 2007 performed MHD simulations including anisotropic heat conduction along the field lines. They simulated the interaction between cold subcluster plasma and hot ambient plasma.…”

“…However, Asai et al (2004Asai et al ( , 2005Asai et al ( , 2007 ignored the effects of plasma viscosity. In the ICM, where the mean free path of ions or electrons is much larger than their Larmor radius and can be comparable to the length scale of clusters, the effects of viscosity cannot be neglected because momentum is transported efficiently.…”

Three-Dimensional Magnetohydrodynamic Simulations of Buoyant Bubbles in Galaxy Clusters

“…Magnetic fields may also play significant roles in the dynamics and thermal conduction properties of "cold fronts", which are contact discontinuities separating merging subclusters (e.g., Vikhlinin et al 2001;Asai et al 2007;Dursi & Pfrommer 2008). That is a similar physics problem in some respects, although strong differences in the initial conditions, relative properties of the media, and the overall dynamics make direct comparisons between magnetic field roles in cold fronts and bubble boundaries nontrivial.…”