Is there a giant Kelvin–Helmholtz instability in the sloshing cold front of the Perseus cluster?

Abstract

Deep observations of nearby galaxy clusters with Chandra have revealed\nconcave 'bay' structures in a number of systems (Perseus, Centaurus and Abell\n1795), which have similar X-ray and radio properties. These bays have all the\nproperties of cold fronts, where the temperature rises and density falls\nsharply, but are concave rather than convex. By comparing to simulations of gas\nsloshing, we find that the bay in the Perseus cluster bears a striking\nresemblance in its size, location and thermal structure, to a giant\n($\\approx$50 kpc) roll resulting from Kelvin-Helmholtz instabilities. If true,\nthe morphology of this structure can be compared to simulations to put\nconstraints on the initial average ratio of the thermal and magnetic pressure,\n$\\beta= p_{\\rm th} / p_{\\rm B}$, throughout the overall cluster before the\nsloshing occurs, for which we find $\\beta=200$ to best match the observations.\nSimulations with a stronger magnetic field ($\\beta=100$) are disfavoured, as in\nthese the large Kelvin-Helmholtz rolls do not form, while in simulations with a\nlower magnetic field ($\\beta=500$) the level of instabilities is much larger\nthan is observed. We find that the bay structures in Centaurus and Abell 1795\nmay also be explained by such features of gas sloshing.\n