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