Magnetic draping of merging cores and radio bubbles in clusters of galaxies

Abstract

Sharp fronts observed by Chandra satellite between dense cool cluster cores moving with near-sonic velocity through the hotter intergalactic gas, require strong suppression of thermal conductivity across the boundary. This may be due to magnetic fields tangential to the contact surface separating two plasma components. We point out that a super-Alfvenic motion of a plasma cloud (a core of a merging galaxy) through a weakly magnetized intercluster medium leads to "magnetic draping", formation of a thin, strongly magnetized boundary layer with a tangential magnetic field. For supersonic cloud motion, M_s > 1, magnetic field inside the layer reaches near-equipartition values with thermal pressure. Typical thickness of the layer is L /M_A^2 << L, where L is the size of the obstacle (plasma cloud) moving with Alfven Mach number M_A >> 1. To a various degree, magnetic draping occurs both for sub- and supersonic flows, random and ordered magnetic fields and it does not require plasma compressibility. The strongly magnetized layer will thermally isolate the two media and may contribute to the Kelvin-Helmholtz stability of the interface. Similar effects occur for radio bubbles, quasi-spherical expanding cavities blown up by AGN jets; in this case the thickness of the external magnetized layer is smaller, L /M_A^3 << L.